Hydraulic oil

The hydraulic oil serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsion, and water-glycol mixtures. The principal requirements 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 frequently used in compact, high-pressure, and high-capacity pumps that require extra lubrication protection. Certain synthetic lubricants and water-containing fluids are used where fire resistance is needed. Synthetic lubricants also are used in extreme-temperature conditions. The classification by performance level is usually conducted according to ISO 6743/4 specifications.

ISO 6743/4 classification of hydraulic oils according to performance level