DOT numbers explination

[1975DCS]

There are more threads about this... But:

Is there anybody that can help in DOT numbers? Audi recommends DOT 4. But some seems to prefer different like DOT 5.1. What are bennefits and downsides?

[4WheelDrifter]

DOT 5.1 has a higher boiling point than DOT 4.
Don't confuse DOT 5.1 with DOT 5 though. DOT 5 is silicone based, and must not be used.

[1975DCS]

Thanks that is simple and clear!

Is it by the way true that some older brake lines and systems can't handle DOT 5.1 because it is to harsh?

[4WheelDrifter]

I know DOT 5 can be trouble because it is silicone based, so it isn't hygroscopic. This can lead to corrosion, siezing and freezing problems.
DOT 5.1 is polyethylene glycol based, the same as DOT 4. AFAIK it's ok to use.
HTH

[dan_b]

Absolutely correct.
(Although, just to add some confusion back in, some DOT 4.0s are better in terms of boiling point than some DOT5.1s, for example ATE Super Blue is classified a DOT4 but far exceeds DOT5.1 specs - not all brake fluids are created equal!)