...Dot4 standard is fine but as it's Polyglycol? based it can over time absorb water so regular changes advised...Dot5/5.1 is silicone based (more £££) but won't ruin paint / wont absorb water / higher boiling point...
Sort of kind of. Actually...
DOT3, DOT4 and DOT5.1 are polyethylene glycol based, which means that they're all hygroscopic - i.e. they absorb moisture into the fluid. The bigger the number, the higher the specification (higher boiling point). The boiling point decreases as the fluid absorbs water, degrading the fluid over its' lifetime. This is why you should periodically replace the brake fluid. Dry and wet boiling points are specified for each of the DOT specifications. AFAIK, there's no major
problem with mixing different polyethylene glycol based fluids, but you should expect performance at the level of the lowest spec brake fluid in the cocktail.
DOT5 is silicone based, and so it doesn't absorb water. There is a school of thought that maintains that this is actually a bad thing, as what can happen is that any moisture in the system will form pockets of water. When the brake fluid temperature gets high enough, these pockets of water will turn to steam which is, of course, more compressible than a liquid. And that's bad news next time you use the brakes. The other issue is that having pockets of water inside your brake system can corrode the metallic components from the inside out, which isn't great. DOT5 should not be mixed with any of the polyethylene glycol fluids mentioned above. I believe it was originally developed for military applications to try and stretch maintenance intervals.
