Originally Posted by warewolf
DOT 3, DOT 4, and DOT 5.1 are compatible, the higher number the higher temperature tolerance but not necessarily "better" depending on other characteristics. You can go higher than specified (eg use 5.1 in systems marked 4) but should not go lower - as Bill discovered when using his bike hard.
DOT 5.0 is the silicone-based stuff and can't be mixed with the others without proper flushing of the system first.
Yup, you get the ceegar! Says right here that you can put dot 5 in a 3/4 system with proper flushing of the system. http://en.wikipedia.org/wiki/DOT_5
There are advantages to dot 5. For those intrested in conversion, read up on the term 'proper flushing' before trying this at home. It also appears KTM has changed the brake fluid spec. sometime after 2000. Thanks guys.