All kinds.  Including:
Mineral Oil-Based Fluids (Hydraulic Oils)
Water-Glycol Fluids
Synthetic Fluids (e.g., polyalphaolefin - PAO, phosphate ester, polyol ester)
Biodegradable Fluids (derived from vegetable oils or synthetic esters)
Skydrol (a brand of hydraulic fluid specifically designed for aviation applications)