Today, practically all types of fluids contain at least one additive, and most have several. It is usually unnecessary and undesirable for the user to put in additive supplements. Probably the most important single property of a hydraulic fluid is its viscosity. Different oils have different rates of change of viscosity with temperature. The use of viscosity index improvers permits the formulation of oil that provides satisfactory lubrication over a wider temperature range than straight mineral oils alone.
Antiwear additives are used to prevent contact in hydraulic components, principally pumps.
Friction is reduced, and surfaces move more freely relative to each other. Wear is reduced under mild sliding conditions. Under severe sliding conditions the additive can be depleted to the point that the wear-reducing effect is lost. When operating temperatures exceed 200 F, catalytic effects of metals, such as iron and copper, become a factor in oil oxidation. Under these conditions, inhibitors that reduce these effects must be used to form a protective coating.