Fluid serving as the power transmission medium in a hydraulic system. The most commonly used are petroleum oils, synthetic Lubricants, oil-water emulsions, and water-glycol mixtures. The essential functions of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are commonly used in compact, high-pressure, and capacity pumps that require extra lubrication protection.
