Synthetic fluid(DOT 5 silicone) becomes compressable when it gets hot, and in a performance application can cause the pedal to go to the floor when the brakes get hot.

Its great for classic cars and such that rarely get driven, and sit for long times with no engine heat to drive the moisture out of the fluid. Many of the higher boiling point racing fluids absorb moisture much more agressivly than street fluid and have to be bled before every event.

http://www.evilplastic.com/bfluid.htm

I use the Ford Heavy Duty DOT 3 as a street fluid.

John C.