In any given lubricant (such as engine oil, transmission fluid or gear oil), there are three main factors that influence the “useful life” of the lubricant: Viscosity Stability, Oxidation, and Contamination. While it is important to understand how these factors affect oil life, it is equally important to realize that none of these factors can be measured or monitored except through a thorough and ongoing oil analysis program. Viscosity - First, let’s examine the oil property known as “Viscosity.” Viscosity is defined as resistance of an oil to flow at a given temperature.