I'd use DOT 4. Should probably say on the cap of the reservoir. Be careful not to let the reservoir go empty, as air can be trapped in the ABS controller and it then requires a computer to be hooked up to cycle the valves to bleed it.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue