I think you can put in 5.1 brake fluid, but there's no reason to. Most great brake fluids people use are DOT 4 but exceed DOT 4 specifications. ATE super blue is good for mixed street/track use, and if you really want something that will stand up to heat, try Motul 600, but change it regularly. Castrol SRF is supposed to be the end of all brake fluids, but it's $70/liter.