Viscosity

The viscosity of an oil may, in simple terms, be considered as a measurement of the fluid's resistance to flow. Industrial oils are classified in terms of their ISO VG rating i.e. the kinematic viscosity (Centistokes) measured at 40 degrees centigrade. Automotive lubricants however, have various international specifications, which may require the additional measurement of kinematic viscosity at -30, -25, -15, -10, -5 and 100 degrees centigrade (SAE rating e.g. SAE 10W/40).

The changes in viscosity found in engine oils are more complex than changes, which occur in industrial oils and depend on a number of effects, which separately or in combination can either increase or decrease the oil's viscosity.

Viscosity increase:

In industrial and automotive oils an increase in viscosity may indicate oxidation, contamination with dirt or water or an addition of a higher viscosity oil to the system. Insoluble levels in excess of 5% weight in diesel engines may cause the oil to become extremely viscous and cause difficulty in starting, blocked filters and oil starvation leading to mechanical failure.

Viscosity decrease:

It is seldom that the viscosity of an industrial oil decreases in use. If this occurs it suggests contamination with a solvent or lower viscosity oil. Fuel dilution in engine oils will also cause a noticeable decrease of viscosity. Degradation of the viscosity index improver may also result in a viscosity decrease.