How does an oil 'lose' some of its viscosity over time and use?

Shear Stability

Shear stability is a measure of whether and how much of the oil’s viscosity is lost during its time in the operating engine. Oil experiences very high stresses in certain areas of the engine. Most multigrade engine oils made from mineral oil contain special types of additives, called Viscosity Index Improvers (VI Improvers). These molecules are larger in size than the oil molecules and are used in small amounts to give better low temperature viscosity while still maintaining an appropriate level of viscosity at operating temperature. However, depending on the size of the VI Improver molecules, as the oil passes through the engine, some of these larger molecules are permanently broken down into smaller molecules that are less effective in contributing to viscosity.

Oils that do not contain special Viscosity Index Improver additives, such as single grade oils or most synthetic oils, will experience little or no permanent viscosity loss. The shear stability of an oil is measured by using both ASTM test methods D4683 and D5275. First, the viscosity of an engine oil is measured. Then, the oil is exposed to severe shearing conditions by repeatedly pumping it through a diesel fuel injection nozzle at high pressure. After shearing the oil, its viscosity is measured again. The percentage of viscosity lost is determined by comparing the second viscosity measurement with the original viscosity measurement.

Although there are no general specifications indicating required levels of shear stability for engine oils, lower percentages mean that an oil is more shear stable and will retain its viscosity better during operation.