by A. J. S. Skinner

An article from a new 'Octabod' with forty years experience in a technical capacity with a major Oil Company and owning a 1933 J2 since 1952.

1. 'Tribology' -the technical name for the scientific study of lubrication, must be a major concern to all of the 'MG tribe' in both their old and new cars, be they petrol or diesel, normally aspirated. Turbo or supercharged.

What you all want to know is; "what oil should I use?" and "what oils are good and what oils are bad for my engine?". The 'good' news is there are no really bad oils on sale in this country. The 'bad' news is that many of the best advertised, expensive oils are not always the best available and rarely represent good value for money. Another common fallacy often exploited by the market is the colour of the oil. Many people prefer the darker green/brown colour of the older oils in the mistaken belief that they are of a better quality than the paler versions now available. In fact, the substances in the oil that give rise to the green/brown colour (nitrogen compounds) actually detract from the oil's high temperature performance as they are easily oxidised. Yet some companies still add green/brown dyes for customer appeal - and as an excuse to increase the price!

We should all pay more attention to the claims stated on the cans and I hope to give you enough information in this article to enable you to translate the 'jargon' into something meaningful. But first, let's consider what an oil has to do in any engine:

1. It must provide a fluid barrier between moving parts to prevent abrasive wear.

2. It must act as a coolant, removing unwanted heat. (25% of engine cooling is done by the lubricant.)

3. It must act as a 'gas seal' in the piston area to prevent the fuel and burnt fuel gases from escaping into the crankcase as 'blow-by' where they can dilute and thin the lubricant.

4. It must prevent corrosion of bearings and components subject to attack by acidic products formed when the fuel burns. (The components in the rocker box are particularly at risk.)

5. It must keep the engine clean by acting as a mild 'detergent' and dispersant to prevent the 'sludge' (caused by fine carbon deposits from burnt fuel in suspension) from clogging the oilways - in other words "engine thrombosis"

6. It must have a 'viscosity' or 'thickness' such that the engine will turn over rapidly when very cold and yet it must still have sufficient viscosity at piston temperatures to prevent metal to metal contact and the resultant wear.

7. It must give a good fill life, retaining its ability to resist combining in the hot engine with oxygen to form varnish like oxidation compounds which can finally become un-wanted sludges.

So, now to the translation of the jargon - perhaps best explained with a little bit of history: In the early days of motoring the refining of oils was in its infancy and there were differences in quality that even engines of those days could recognise. The earliest effective attempt to rationalise lubricants was made by the American Society of Automotive Engineering (S.A.E.) who nominated a viscosity or 'thickness' range of lubricants - this at least meant the viscosity was controlled. Viscosity was thought then to be an oil's most important factor. Thus was born the familiar S.A.E. rating system for engine oils.

S.A.E. 10, 20, 30, 40, 50 and 60 were the original grades, (the higher the number, the thicker the oil). Later, two further classifications were introduced; a maximum thickness on viscosity was imposed for low temperatures to assist cold starting and a 'W' (for 'Winter') suffix was added to oils that passed this specification. Thinner oils were also now classified, resulting in a range of S.A.E. 5W, 10W, 20/20W, 30, 40, 50, and 60.

The degree by which oils reduced in viscosity, that is, become 'more fluid', as temperature increased was then investigated resulting in the Introduction of 'Viscosity Index' or V.I. An oil which tended to lose a large percentage of its thickness with temperature increase had a poor Viscosity Index rating and those which maintained viscosity were referred to as High V I oils.

In the 1950's it was found possible to boost Viscosity Index by adding long chain molecules (polymers). These dissolved as temperatures rose and combated the reduction in viscosity. This discovery meant it was now possible to produce oils which spanned several S.A.E. numbers - the Multigrade had arrived!

So now we had oils which were thin enough to qualify for a S.A.E. 10W rating at the low test temperature and yet could maintain a thickness rating of SA.E. 30 oils at engine working temperature. Multigrades are now the 'norm' be they 10W/30, 20W/50, or more recently 15W/40 or 10W/30.

Early Multigrades used very long chain polymers which, quite literally, got 'chopped up' by gears into smaller molecules losing their 'viscosity boost' characteristics. This fault was termed 'Polymer shear'. Modern oils use a much larger addition of more stable, shorter chain molecules, not so prone to shear and will now stay 'in grade' for the oil's fill life - now up to 12,000 miles in petrol engines. But remember, viscosity ratings only tell you about the viscosity, they do not tell you about the other properties which the engine needs and consequently they are now considered to be less important than the oil's engine performance data.

Different engines have different problem areas to lubricate. For example, a 'Volvo' unit will have heavy valve and tappet pressures whereas Perkins Diesel Engines will generally need an oil capable of suspending a fairly high percentage of fuel soot. Major oil companies must aim their products to satisfy all engine demands so - 'additives' are used to enhance the base oil's performance. Fortunately, most oil additives perform more than one duty.

In an attempt to quantify 'engine performance data' the American Petroleum Institute (A.P.I.) has devised a series of complicated, and thus expensive, tests to rate any oil's performance in service. The result, known as the A.P.I. Performance Level, is now regarded as the oil's most important aspect. Viscosity has been relegated to second place. The A.P.I. specification is the one you should be looking for.

The classification can best be explained by a simple diagram:

Compression Ignition
CA           SA
Spark Ignition

The further away from the centre line the classification appears the more severe has become the test procedure. The initial 'C' is used to denote 'Compression' or Diesel engine whilst the initial 'S' denotes 'Spark' ignited Petrol engines. These tests cover every aspect of an oil's performance in really scientific detail. The second initial is alphabetical so that, for example, the next specification for diesel will be CE and for petrol SH when they are devised.

The tests are usually updated about every four years and once a new specification has been introduced, you can no longer gain approval for products meeting the out-dated specification. If the oil you buy is not the latest A.P.I. specification and the producer has had oils properly qualified before, he may be allowed to quote his oils as being of a certain A.P.I. level but he may not say that it is qualified unless it has actually passed the relevant engine tests.

Petrol engine oils of top quality today will need to cope with the fact that crankcases are no longer permitted to evacuate directly to atmosphere. 'Positive Crankcase Ventilation' (P.C.V.) now adds a lot of stress to the lubricant. The use of turbochargers further stresses the lubricant. The latest relevant petrol engine specification is A.P.I. - SG. For diesel engines, including turbos, it is A.P.I. - CD. So, if you look on the can of a 'top' product you should see "qualified to A.P.I. SG/CD". I doubt if you will be able to buy now an oil with a lower treatment level than A.P.I. - SE/CC from normal sources.

Although your pre-war MG would be satisfied with A.P.I. - SB quality use of material well above this level will be an advantage so long as the additives' used have low 'ash values'. (The residue left when the lubricant burns which can plate out on valve heads.)

Reputable Oil Companies use 'low-ash' additives. Oil additives have become a very scientific business. I don't think any oil company now produces all the additives it uses, but buys in from specialists who rate their 'additive packages' by expensive engine testing. So you see, the old saying "it is a wise old cow that knows her own tin of milk" is true in the oil world too. So, what should one use in a pre-war MG -say up to TC models, where valve stem seals were unknown, or at best poor? Oil consumption is usually relatively high and often there is no air filter or oil filter fitted. (My J2 is a good example.) Contaminants are therefore circulating with the oil.

I would use an oil of at least A.P.I. SE quality, of S.A.E. 20W/50 viscosity rating and I would change the oil at least every 2,000 miles unless the car is in daily use. In the more modern MG's, I would be happy to use oil of A.P.I. - SF minimum quality of S.A.E 15W/40 rating and change about every 3,000 miles on predominately short journey use or up to 5,000 miles if the car is used mainly for long runs. (Long runs help the oil to distil off any fuel contamination thus improving the oil's viscosity and reducing its volatility and usage.

Where should I get the oil?
The major motor service agents such as 'Ford Motorcraft', 'Unipart', 'GM' etc., all stock oils which tell you their quality on the can. They will have been made by one of the major oil companies. You just won't know which one. If the can doesn't tell you the A.P.I. rating then frankly - don't buy.

Market leader brands are usually the most expensive and often try to justify the price by claiming to 'exceed' rather than just 'meet' the required specifications, - a statement you can neither prove or deny. Remember, the engine manufacturer is the one who has to nominate the lubricant's specification for his engine, so as long as it satisfies his specifications(as the A.P.I specification will do) he, the maker, can find no advantage in a possible 'over specification' material or it would be to his advantage to advise its use.

'Oil Consumption' and 'Fuel Economy' are both complex subjects which I may well explain in a later series if there is sufficient interest. The same remarks also apply to 'oil and air filtration'.

If I was not worried about the price I was paying, I would buy anyone of the well known forecourt grades and feel happy. If I were running a diesel engine I think I would always purchase A.P.I. - CD quality even if I didn't have a turbo-charger fitted -and an oil of S.A.E. 15W/40 seems ideally suited in diesel engines changed at the manufacturer's recommended oil change intervals -- usually upwards of 3,000 miles

If I were (and I am) running a modern Ford or any turbo charged petrol I unit - I would be sure to use A.P.I. - SG in order to avoid 'black sludge' seen in the rocker box -today's major oil 'baddy'. S.A.E. 15W/40 would give good oil consumption, good fuel economy and a fair degree of 'insurance' against fuel dilution. All modern engine oils are mineral oils and are fully miscible with one another. Vegetable based oils such as castor oil have a good load bearing performance and are soluble in alcohol based fuel for 'tuned-specials' but they are very poor in high temperature oxidation resistance and are now only available to special order . The only thing they have now over mineral oils is the nostalgic 'odour' of the exhaust gases.

Some Do's and Don'ts
1. Do drain oil only when it is hot and contaminants are still in suspension.
2. Top-up oil only when hot to avoid over filling, the latter reduces engine power and increases oil consumption. The practice of checking and topping up a cold engine before a journey usually results in over-filling. (Yes, I know many manuals advise checking before a journey!)
3. Change oil and air filters regularly. If you don't have either, then think of fitting an air filter; strangely, it is more important in today's driving with older cars than is an oil filter.

Next time then, some information on gear and transmission oils, greases and specialities. Among the 'specialities' I will include Fluid and Colloidal DIY Oil and Fuel Additives, synthetic or selected hydrocarbon (S.H.C.) motor oils; the 'rash' of 'Vintage-style Blended' Oils said to be the last thing for older designed cars Remember, oil is still amongst the lowest of your motoring costs despite the fact that, like your fuel, it has probably travelled half way around the world before you use it.




Strange to relate, the base oil in gear lubrication is the same as that used for engine oils. In fact, the base oil doesn't know what kind of oil it is going to be until the 'additives' are put in, to slant its use towards engine, gear or hydraulic lubricant etc.

Most Spur Gear sets will run quietly on an oil of a viscosity as low as an SAE 30 engine oil. Spiral bevel gears will usually run best with an oil of SAE 50 rating and will benefit from some Extreme Pressure (EP)additive treatment. Hypoid Gears however will need a more chemically active EP agent as the oil has to cope with rolling and sliding motion because the axis of the pinion is below the crown wheel centre line. Limited slip differentials will also impose EP requirements that need to come into play at relatively low temperatures, The faster the gears run, the thinner the oil that can be used. Slower running gears need thicker lubricants which are still able to coat the teeth when they are not immersed in the oil bath prior to their next contact.

Just as in engine oils, the viscosity is specified, but the test temperature is not the same as it is for engine oils. A test temperature of 140degrees F (60 C) was selected and was considered to be the normal 'steady' running temperature of gear units. It can take up to two hours to reach this 'steady temperature' in a rear axle, and this is one region where multigrade oils can help to save fuel. Originally SAE 80, 90, 140 & 250 were designated. As the numbers increased so did the viscosity and in fact they doubled in viscosity each time -- thus an SAE 140 was twice as thick as an SAE 90 and four times as thick as an SAE 80.

The range has now been extended, although SAE 250 is not normally available in gear oils, nor is it normally required. The most popular ranges now are 75W/90 and 85W/140. There was unfortunately, a period when many oil companies sold 90/140 which looks like a multigrade but isn't. Its just a thick SAE 90 or thin SAE 140 however you prefer to look at it. The limits for the SAE 90 and SAE 140 overlap, it could be both grades technically -- a deceitful way of marketing, I am sorry to say, used to advantage by most major oil companies at some time. Present 75W/90 and 85W/140 oils exhibit multigrade properties.

Modern hypoid oils use additives which have a minimum corrosive effect on bronze or brass gearbox components so it can be used in both the gearbox and rear axle. The index of performance is the 'G.L Rating. GL-3, GL-4, GL-5 and GL6 are obtainable. GL-3 is light EP treatment. GL-4 Hypoid service level - the most commonly found in practice, GL-5 is Hypoid 'initial fill' quality used in the running-in period and GL-6 is limited slip differential oil.

In my experience pre-war and post-war M.G.'s run well with hypoid GL-4 of 85W/140 viscosity in both rear axle and gear box. Some motor manufactures, because of possible corrosion, still dislike the use of hypoid oils in gear boxes (in particular, Vauxhall and Volvo) so it is wise to check your manual. My J2 has run for years on Hypoid 90 in rear axle, gearbox and steering box and is now on the more modern 85W /140 hypoid lubricant (GL-4 sequence).

Ingress of water to Hypoid oils can give a very serious rust problem. Most Hypoid oils use chlorine and sulphur EP additives which, if wet and at high temperatures, can produce strong corrosive acids.

More really modem cars (post 1980) specify a Hypoid oil of GL-5 sequence and this one oil charge is used for the life of the vehicle - not being changed at all. This 'fill for life ' system is on the whole a very good idea since changing gear and axle oils often leads to dirt getting into the system during the change and doing more damage than if the old oil was left in. So, when you are changing oils, thoroughly clean and dry the fill plug area before you start.

The build up of debris inside the system will depend on the transmission design which will require the oil to be changed at the manufacturer's recommended period - in early M.G.'s as often as every 2000 miles - so consult your manual. Overfilling transmissions wastes power and can ruin oil seals, so check oil levels with the unit hot. Since test temperatures for engine and gear oils are not the same, engine oils can also have a gear oil viscosity rating. An SAE 30 engine oil is an SAE 80 gear oil, SAE 40 and 50 engine oils are SAE 90 gear oils, but of course they can only be used (and are often specified) where EP treated oils are not advised. Usually because of bronze synchromesh components.

These vary vastly depending on the type of system in use. Most automatic gearboxes are of the epicyclic type and revolve fast, so an SAE 10W viscosity rating suffices. The frictional characteristics of the oils are critical for smooth gear selection. Often, rubber seal swelling agents are formulated in the oil, and with Automatic boxes it really pays to use only the manufacturer's recommended type to obtain a smooth gear change sequence.

These are really chemicals and are produced only by about four company groups world-wide; the oil companies and brake system manufacturers buy from them. The boiling range of the fluids is important to minimise vapour pressure problems in the hot running parts of brake systems. Most systems must also protect against water vapour being absorbed by the brake fluid thus altering its characteristics and causing corrosion.

Most units will use fluid to SAE - 7OR3, D.O.T. 4 or D.O.T. 5 specifications and any fluids meeting these specifications are fully miscible with one another - that is a clause in the specifications. However, one brake company still recommends only its own fluid and makes 20% of its profit from brake fluid sales - very emotive things are brakes! Brake fluids have very low temperature fluidity and high temperature oxidation resistance, so it's not surprising they are expensive. The latest silicone fluids are excellent brake fluids and offer virtual 'fill for 'life' benefits. They don't absorb water or moisture to corrode the system and they are not 'paint strippers' either, but they may cause components to swell!!

The old name for grease was 'solidified oil' and that really was a good name. The addition of an alkaline soap to oil can result in a 'gel' with good lubricating qualities. Greases have been made with calcium, sodium, aluminium and lithium soap bases, each having different characteristics. Today's greases are nearly all lithium complex greases and they are superb, being fluid at very low temperatures and yet not running away at high temps. They now incorporate additives to improve their oxidation resistance and give metal adhering 'tackiness'. Greases are measured for 'firmness' and the National Lubricating Grease Institute (N.LG.I.) rating is:- 00-0-1-2-3-4-5. The higher the number .the 'thicker' the grease. For today's use an N.LG.I. number 2 Lithium complex grease is all you need bother about. It will do all the jobs from hub bearings to chassis lubrication. Greases really have come a long way in the last 20 years. The use of a grease gun will evacuate the old grease and provide a suitable new grease charge.

Colloidal forms of graphite, molybdenum di-sulphide and copper have certain advantages in lubricants. In my view, the inside of the internal combustion engine does not benefit from this type of lubricant, but use of molybdenum disulphide does improve road salt corrosion resistance in greases and copper powder certainly works well in greases and as an anti-seize additive. (For bolts, NOT pistons!) Colloidal solid lubricants dispersed in oils including polytetrafluoroethane - P .T .F .E. -- the most slippery substance known -- are well advertised.

Solid film lubricants only come into play when fluid lubrication breaks down, so for a start, this makes it difficult to evaluate the additive. The important aspect is the 'colloid'. A colloidal particle is so small that it is affected very little by the force of gravity and it stays suspended in a fluid. Milk is a good example of a colloid, i.e. fat in a water base.

Colloids are usually produced by continuously rolling balls of the compound in a drum containing the fluid medium. Unfortunately, the process is lengthy and as sales of the product increase, so the drum rolling time is reduced to cope with the demand; the product then has inferior colloidal characteristics and is consequently easily removed by the engine's oil filter.

P.T.F.E. of which great claims are made, is difficult to get into colloidal form and anyway is unstable at temperatures above 350 degrees F. These temperatures are commonly met in the piston area and acidic chlorine compounds produced are likely to radically increase bore wear. Many anti-wear additives are on sale for addition to your motor oil - all make claims that are pretty impossible to check. In most cases, the addition of these products is not harmful but it is expensive. Additive balance is an important part of producing a quality oil passing the A.P.I. tests. Increasing say the anti-wear property may well produce an oil with an inferior rocker box corrosion performance to such a degree that it could not be A.P.I. qualified.It is always cheaper and more effective to buy a top quality oil rather than purchase additive boost packages.

However, the motoring world is full of people who like to spend a lot of money in the belief that they can improve on products the oil companies have spent a lot of time and money perfecting. Experience tells me that you could package camel's urine in a pretty tin and charge at least 3.50 a time and it would sell even if you admitted it was camel's urine!

The oil chemist is now able to 'construct' oils from selected molecules which are known to give better viscosity characteristics and superior oxidation resistance. These 'synthetic lubricants' as they are termed, have been used for years in gas turbine engines. They are also ideal for use in high performance air compressors where they can usually quadruple the normal oil change life. However, the pollutants from products of combustion in a normal car engine must negate the effectiveness of any synthetic lubricant, reducing its cost effectiveness. Modern cars, however, are ideally suited to synthetics.

On the continent of Europe synthetics are the vogue and in some countries are the only oils sold on the forecourts. Such oils are expensive but offer the oil companies higher percentage profits than normal mineral oils, but are ideal in modern cars. No M.G.'s were designed to need synthetics although, providing they are of suitable viscosity, there is no reason, other than cost, to avoid them.

There has been a big increase in the sale of so called 'vintage car blended oils'. Several companies are producing oils claimed to be 'blended as oils used to be' and they are selling at really very high prices for what they are. Some companies have gone so far as to quote S.A.E. numbers like 40/70 or 30/70 - CLASSIFICATIONS THAT DON'T EXIST! When I pointed out this 'error' to one of the retailers, he said; "well it is only to give you an idea". - now do you think it was really for that reason or was it an excuse to 'up the price' - I know which I think! Oils of this type of viscosity may show advantages in 'veteran' cars but all they will do in M.G.s: Is to use up power and increase fuel consumption - for no good reason. The engineering used in even the earliest M.G. was well above that used in most veteran cars.

Lubrication normally aimed at in engines is of the Full Fluid Flow type or technically 'Hydrodynamic Lubrication'. In this case the moving member, such as a shaft revolving in a bearing, is producing, due to its rotation, an 'oil wedge' with load bearing ability. Let us consider a shaft at rest in a bearing housing which it cannot fully fit. The shaft will be in contact with the bearing due to gravity which can be diagramatically exaggerated as:

Shaft at rest

Now, as soon as the shaft starts to rotate,

rotation starting

the presence of friction will cause it to climb up the bearing face, compressing any fluid in the bearing cavity and causing an oil wedge and a tangential force which will lift the shaft out of bearing contact. The shaft will then rotate freely just below the true centre of the bearing once again due to gravity acting on the shaft;

rotating freely

Under these conditions, wear is negligible and due to the shaft's rotation the 'apparent viscosity' of the oil in the load bearing wedge is very many times that of its real normally measured viscosity, Thus, once motion starts, bearings do not know the difference between an SAE 10 and an SAE 50. So far as bearings are concerned then, there is no advantage in high viscosity oils.

When it is not possible to provide Full Fluid Flow for component separation an intermittent contact of moving parts will result. Obviously, some wear will occur. Under these conditions, the viscosity of the lubricant is critical - the load bearing property is then directly proportional to the oil's viscosity. The thicker the fluid then the lower the wear rates. Pistons are of course a good example of boundary lubrication, being accelerated rapidly from and to rest at each stroke.

Microscopically, surfaces are never truly smooth and two planes in contact will always actually rest on the high spots. The more points in contact the lower the unit force on each - this is what we aim for when running-in. Where contact occurs, temperatures rocket and the metals quickly weld together. The momentum of the moving parts tears the welds apart and wear occurs with poor surfaces produced. The use of certain chemical elements such as chlorine, sulphur or phosphorus forms the relevant iron chloride, sulphide or phosphate at the weld due to the high temperature. Metal soaps will separate from one another at much lower force than true metallic welds and surfaces coated with metal soaps will show a reduction in friction between them. We can therefore obtain limited wear and a protected surface finish by using anti-wear and extreme pressure additives. This type of lubrication is commonly found in the rocker box cam and tappet operation and particularly in hypoid gears. The viscosity of the oil plays a part but the chemical treatment is the most important element in successful lubrication of this type.

You can see then that lubrication of your engine is always a compromise to try and produce low wear rates in all parts of the engine under all conditions.

Given time, the oil industry has always managed to meet the engineers' needs. In 1940 during the Battle of Britain we were losing many Merlin engined aircraft due to piston seizure, when pilots over used the engine boost, and were being shot down by the enemy. At the piston temperatures experienced the lubricant in use was so thin it could no longer cope. The use of a higher SAE grade would have meant that in the cold start situation the oil drag would have not allowed the engine to crank fast enough to start. In America a special type of refining was available which gave base oils of very much higher viscosity index and this product did the job and may well have 'saved the day'.

When Concord was built, it was necessary to lubricate control linkages that would be used in both the high temperatures of Saudi Arabia and the very low temperatures at high altitudes. Lithium complex greases provided the answer.

We can now use the same lubricant in diesel high performance engines as we use in our petrol engines. A useful rationalisation.

Synthetic oils have progressed, become cheaper and, since they offer extended fill life, will make environmental sense.

The oil world is a fascinating, fast moving market and I don't doubt the oils of the 21st Century will be even better all round.

A. J. S. Skinner (1991, updated 2002)