The viscosity of a liquid is its resistance to flow and is directly affected by the temperature of the liquid. An oil's viscosity decreases with increase in temperature and increases when the temperature is reduced. Kinematic viscosity is expressed in centistokes (cSt) and is usually measured at 40°C, but can be measured at 100°C as well
Increases in viscosity may indicate the onset of oxidation of an oil but may also be caused by residual fuel leakages, or an admixture with a heavier grade of oil. The most common reason for a viscosity increase is the build up of the quantity of total insolubles present in the oil.
Decreases in viscosity are usually due to distillate fuel dilution but may also be caused by an admixture with a lighter grade of oil. Contamination by water may also change the viscosity of an oil if emulsions are formed.
The maximum increase, or decrease, of viscosity tolerated is usually taken as ±25% of the viscosity of new oil, or when the viscosity of the oil has increased or decreased into the adjacent SAE grade.