To use or not use DOT5 fluid?
No, they do not all mix. DOT 5 is it's own animal. It's made of a synthetic material and cannot be mixed with DOT 3 or 4. If mixed you will loose the temperature resistance, its anti-corrosion properties, and it will turn a cloudy color.
If you wanna go with something a little better the DOT 3 without having to completely remove all the DOT 3 from the system, (and I mean all, not even a drop of it,) I would recommend going to DOT 4. You can mix with DOT 3 although you will want to get most if not all of the DOT 3 from the system. DOT 4 has a higher boiling point than DOT 3 does, hence removing as much of the DOT 3 as possible.
