Shimano has been making hydraulic brakes since the early 1960's and 70's, first seen in a rim brake application. At some point in the 70's the hydraulic brake system came with a caliper massive enough to probably stop a moped. The original service manual from the 70's can be viewed below, it's interesting to see how things have developed over the years. Shimano hydraulic mineral oil  is formulated for the proper maintenance of Shimano hydraulic disc brakes - Shimano recommends that you use only genuine Shimano hydraulic mineral oil in Shimano hydraulic disc brake systems. Mineral oil brake fluid is environmentally safe and non-corrosive.