Certain physical and chemical characteristics of petroleum-based hydraulic fluids are determined by the base stock, while others are due to additives. Some additives impart useful properties, some enhance properties already present, and others act to reduce the rate at which undesirable changes take place in service. 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.