When the lubricant is used, three main processes influence the viscosity and the aging process:
The mechanical destruction of high-polymer VI-improvers causes an immediate decrease of the viscosity and, in any case, a deterioration of the viscosity-temperature (VT)-relation.
At the same time, there is a process of thermal oxidation which – depending on the temperature range that the liquid is subjected to – may lead to a continuous rising of the flowability. This effect is supported by the growing exhaustion of the specific oil additives.
As the lubricant is being used, there is always an appearance of a certain amount of particles whose type and quantity depend on the specific application. While they may be external impurities or the result of friction or of combustion, the consequence is always an increase of viscosity.