Some properties of hydraulic oil are foaming resistance and air release, anti-wear performance, filterability, demulsibility and rust and corrosion inhibition, according to Machinery Lubrication. Additional properties are thermal, oxidation and hydrolytic stability, high bulk modulus and high viscosity index.
Foaming resistance is a measure of how much foam is contained in the hydraulic fluid. Air release is how quickly air bubbles escape out of the liquid.
Filterability is the fluid’s ability to pass through a filter without an undue drop in pressure, while demulsibility is the oil’s ability to release water.
Rust and corrosion inhibition is the use of additives in a fluid to counter its potential to rust and corrode, explains Hydraulics & Pneumatics. Thermal, oxidation and hydrolytic stability is a fluid’s ability, with the aid of an additive, to exhibit thermal stability, inhibit oxidation and neutralize acids.
High bulk modulus indicates that hydraulic fluid exhibits a high resistance to compression under pressure. High viscosity index means that the fluid has a low rate of change in viscosity with temperature.