Originally Posted by
CoRDiTe
Dot 3 and 4 have different boiling points for wet and dry.
yeah, that's due to that added borate esters. different amounts make for different boiling points.
since the base is the same, it is safe to mix the two. You'll just, basically, average the two boiling points together.
Originally Posted by
CoRDiTe
Dot 5 iirc the chemicals in it cannot be used with vehicles that have ABS systems.
DOT5 is silicone based. the problem with silicone is that it compresses slightly under pressure. Because of that, when the abs pump does its thing, it compresses some, allows for air space, then expands back, and repeat. After a few cycles of that, you've created foam, and as you can imagine, that isn't good for ABS systems.
Now,
DOT5.1 is a different story. DOT5.1 starts its life out as DOT4.
DOT5.1 basically has a higher boiling point, but lower life span since it absorbs water more quickly. There are a few other differences too. Generally speaking, unless the manufacturer recommends DOT5.1, there really isn't any reason to use it since there are many DOT4 fluids that have very high boiling points, wet and dry. DOT5.1 would really only be useful in super crazy brake systems on purpose built cars where you only need the fluid to last through a race or two, and then the whole system gets flushed.