mineral oil is the official substance meant for it,there are a few offering about from shimano to monty and each feels different so experiamentation can be done.i've also heard of cooking oil being used but meh to that. water is also used alot as it give a very light feel to the lever and is inexpensive to use,but. . . it voids warenty, occasionally it damages seals in the brake wich isnt good and it requires antifreeze during winter to stop your brakes from icing up .