One of the most important factors when selecting lubricants, defined as the number of seconds it takes for a set quantity of a sample to pass a certain length at a certain temperature. Various units can be used to measure viscosity depending on the measuring instrument, but the most common is “centistokes” (cSt). Furthermore, viscosity changes with temperature, so the temperature must be specified for the viscosity level to be meaningful. As conditions such as speed, temperature, and load vary for each device, they all require lubricant viscosities. If the viscosity of the oil used exceeds the required viscosity, it may cause damage due to excess operation, but if the viscosity is too low, the oil cannot form the required oil film, and scratching or other such problems may occur. In order to measure viscosity at cold temperatures other than by using ASTM D-445, the ASTM D-2602 method with a cold cranking simulator can be used. The test standard used to measure the viscosity of GS Caltex engine oil is ASTM D-5293.
The viscosity index shows the relationship between the oil's viscosity and temperature. When the temperature rises, oil viscosity decreases, and vice versa. A high viscosity index means that temperature changes do not greatly affect viscosity.