Leading hydraulic system designers regard hydraulic fluids as the single most important group of materials in hydraulic systems. People in the industry believe that fully 85% of all hydraulic fluid ever installed leaks out: either slowly or in major line breaks or failures of fittings, seals, and the like. The widespread use of the term hydraulic oil reflects the dominance of petroleum-base hydraulic fluids. A hydraulic fluid serves not only as a medium for energy transmission, but also as a lubricant, sealant, and heat transfer medium. It must be designed to maximize the efficiency of the hydraulic system, which means more power applied with less wear and tear to the system. Viscosity measures a fluid’s resistance to flow. The fluid’s viscosity must be high enough to ensure a full fluid film between moving surfaces, but not so high that it creates excessive frictional drag. If the resistance to flow is too great, excessive fluid friction develops within the system.