Viscosity
What is viscosity?
Many people have the misconception that SAE numbers are a continuous series, and that the higher the SAE number, the higher the viscosity. They are unaware that, in general, the viscosity of SAE 30 motor oil is similar in viscosity to SAE 90 gear oil. Another point of interest is that the minimum viscosities for both the SAE motor oil series and SAE gear oil series are essentially the same: 30 SUS @ 210°F for motor oil and 40 SUS @ 210°F for gear oils. It would seem logical to ask why motor oil and gear oil cannot be interchanged, or the two systems combined into one. The real difference between engine oils and gear oils is not viscosity, but additives.Viscosity can be defined simply as the internal resistance of a liquid to flow. Viscosity is stated relative to a given temperature. Tests are generally done at 100°F and 210°F.
Viscosity Index (VI) is an empirical number which indicates the rate of change in the viscosity of an oil within a given temperature range. For example, a high Viscosity Index oil thins out at elevated temperatures slower than a low VI oil. The advantage of VI rating is that it capsulizes the effects of temperature as a single number.
An increase in the VI of an oil blend can be brought about by the incorporation of a VI improver. VI improvers are used when the properties of an oil make the desired high VI impractical to obtain by solvent refining, and when the applications are subject to extremely wide temperature ranges.
Viscosity functions as a part of the everyday language of lubrication. When considering specific application problems, it is helpful to have an understanding of the physical properties a lubricant will exhibit at a given operating temperature. The tendency toward simplification in lubrication leads to the use of oils that satisfy a variety of applications. Therefore, decisions about the most satisfactory oil are based, to a large extent, on viscosity. Oil viscosity in both fluids and greases remains the initial concern in making a good lubricant recommendation.
The T.S. Moly oils described in this section are developed from solvent refined mid-continent sweet crude. The Viscosity Index ratings are indicative of the excellence of the products.