Mineral based…does that include synthetic?
Well here is to some new egg on my face. Going off to do a little learning is apparently a good thing…if embarrassing.
There are really three types of brake fluid generally used. Glycol based, silicone based and mineral oil based.
I have always heard and used the term mineral based for the DOT fluids which is apparently not at all mineral based. DOT fluids are glycol based and is hydroscopic which means it will absorb water and degrade the fluid’s boiling point. However the water is bound so it is less likely to damage the system components (at least as long as it is replaced in a reasonable time period, which by manufacturers recommendation is two years though I am sure many of us fail miserably on this). Both silicone based and mineral oil based fluids do not absorb water however any water in the system will tend to pool in the low areas of the system and cause rust or other corrosion to the system.
In all of these cases the entrained water will degrade braking performance due to the relatively lower boiling point of the water creating compressible steam so even with the hydrophilic fluids one does need to change them. The good thing about the silicone or mineral based fluids you can reuse the liquids one the water has been separated out.
The DOT glycol based fluids have to be discarded as the fluid has been chemically degraded.
Anyway, ‘standard’ or ’synthetic’ DOT glycol based fluids can be mixed. Just not with the other two types of fluid.
Whew…
I know you are asking yourself what uses mineral based oil brake fluid. Mountain bikes, some motorcycles and some old Rolls Royce/Bentleys and unsurprisingly, Citroen (and Maserati for a while when they were part of the Citroen sphere).