dot 4 (glycol based, boils at 230C)
dot 5 (silicone based, boils at 260C)
dot 5.1 (glycol based, boils at 270C)
Dot 4 & 5.1 are both glycol based and therefore hygroscopic.
Dot 5 is a silicone blend, and is not hygroscopic, but can cause premature wear in seals, as moisture is forced into the rubber or plastic. They are also more compressible.
Mixing DOT 4 & 5.1 is fine, but if yor wish to consider a silicone blend, you will need to flush the system, or you will experience a chemical reaction causing solids to precipitate out of the mixture and clog up your braking system.
Last edited by 007 : 29-Jan-2009 at 05:51 PM.