Mercedes brake fluid must be DOT 4+.

Brake fluid is another place where people can blow a lot of money for absolutely no good reason. Wet boiling points are what you should be looking at, higher is better. There are several brands of "racing fluid" like Motul, Pentosin, Redline.

But the general rule is this: Regardless of brand, the best fluid is new fluid. Fluid should be changed right before racing.

The consequences of not doing so generally suck:
http://www.youtube.com/watch?v=StyU5QdmXlc

The effect is usually called "Losing My Brakes" but almost 99.9% of the time this is due to people doing a track-day on old brake fluid.