Good question Patman, Temperatrue is a major factor when discussing viscosity of an oil. Visc has an inverse relationship with temperature Visc decreases with increasing temperature (the oil thins out) and rises with decreasing temperature (the oil thickens). The rate of change of viscosity with temperature change can be dramatic. Visc's are usually measured at certain standard temps, such as 40 and 100 degrees centigrade or celsius, which have proven useful in the selection of oils for different applications. In use, an oils visc varies depending on the temp at various locations in the lubrication system of the equipment. For example, it would be different in the piston ring area of an engine than in the main bearing area. So, the most common means of discribing an oils visc/temperature relationship is with an empirical number called the "viscosity index" or "VI". Again, the relationships are inverse. The lower the VI, the greater the change of visc of the oil with temperature.