I would like to throw in my knowledge and experience. I used to develop brakefluids for a major global chemical company from 1980 until 1985. Many major oil companies, brake component and car manufacturers used to be our customers. We sold this business in the meantime,but the technology hasnīt really changed.
DOT 3 and DOT 4 are both based on glycolethers, in fact DOT 3 fluids are straight glycolether based and DOT 4 fluids are typically based on borate esters of glycolethers. DOT 5 are silicon based fluids. This is globally valid.
So none of the fluids are mineral oil or vegetable oil based. The only cars that use mineral based brake fluids typically have a central hydraulic system, like some Citroen models for instance. Filling brake systems that are designed for DOT 3 or DOT 4 with a mineral oil based hydraulic fluid will lead to catastrophic failure, due to extreme swelling of the rubber seals.
DOT 3, DOT 4 and DOT 5 are supposedly interchangeable among eachother as far as their rubber compatibiliy is concerned. I.e. all 3 should be compatible with the rubber seal materials that are designed for conventional brake systems. In order to meet the DOT 3, DOT 4 or DOT 5 specs. these brakefluids are formulated such that the swelling of rubber components is within limits, that are clearly specified in the DOT 3,4 and 5 standards. Like wise rubber seals for conventional brake systems, should stand any of the DOT 3, DOT 4 or DOT 5 fluids.
Where things can go wrong is when one mixes a DOT 3 or 4 fluid with a DOT 5 ( silicone based ) fluid. Let s assume initially a DOT 3 or 4 fluid had been filled into the brakesystem. The seals are īsaturatedī with DOT 3 or 4. Now a silicone fluid is filled. If the absorption of the silicone fluid is faster than de desorption of the DOT 3 or DOT 4 fluid, then excessive rubber swelling will occur. I.e. the solubilities and the rates of absorption/desorption can be different and that leads to the reported problems. In general it must be recommended to change rubber seals when canging over from DOT 3 or DOT 4 to DOT 5.
A few words about preference for DOT 3 or DOT 4
DOT 4 fluids have a somewhat higher boiling point than DOT 3, when fresh. Also
with the same amount of water absorption ( throgh the rubber brake hoses and
seals as well as into the brake fluid reservoir), the DOT 4 fluids show a higher
boiling point than DOT 3 fluids. However DOT 4 fluids are more hygroscopic and
tend to absorb water faster than DOT 3 fluids. I.e. after a while they become
equivalent in boiling point, and the DOT 4 fluid will have a higher water percentage
at that point ( possibly more corrosive). For normal use and if brake fluids
are regularly changed ( once every 2 years) then DOT 3 fluids are just as suitable
as DOT 4 fluids. A DOT 4 fluid, which is not regularly changed is worse than
a relatively fresh DOT fluid: more water absorption means lower boiling point
and more corrosion.
A few words about DOT 5
DOT 5 fluids have been developed for military applications, i.e. for vehicles
that could stand in storage for years, without maintenance, yet had to perform
immediately when required. They are superior in terms of boiling point retention
and corrosion / conservation properties, since they don t absorb water.
It may be better today, but one of the big drawbacks used to be their low air solubility leading to a spongy brake pedal feel. Other big advantage of course is that silicone fluids don t dissolve car paints.
Summary:
-- DOT 3 and DOT 4 fluids are fine if they are changed regularly.
-- DOT 3 is certainly not inferior to DOT 4 for normal use.
-- DOT 4 can be inferior to DOT 3, if DOT 4 isn t changed regularly.
- - if changing from DOT 3 or DOT 4 to DOT 5 or vice versa, it is recommended
to change all rubber seals.