Ive used loads of stuff. But water seems to work the best. Sounds daft but it does. Gives a sharper lever feel. But to be honest it's best sticking with Magura Royal Blood, because hydraulic brakes need lubrication and water is not the best lubricant. The safest option is to use Magura royal blood. But theres other's that work. I've used WD-40, DOT 4, Royal Blood and water, and water is far the best and easiest. Oli x