Mineral Oil

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. 

Although Shimano made DOT fluid brakes in the 1960's and '70's, our modern brakes use mineral oil. Mineral oil won't harm painted finishes, and is less damaging to your skin and the environment. Chemically speaking, DOT fluid absorbs humidity from the air, and this absorbed moisture lowers DOT fluid's boiling point over time. Shimano mineral oil starts at a higher dry boiling point than any DOT fluid specification. The boiling point of mineral oils won't change over time because they don't absorb moisutre.