The physical properties of motor oil are viscosity, pour point, flash point, NOACK volatility, acidity and alkalinity. These properties vary based on the grade of motor oil and the petroleum base stock from which it is derived. They also reflect the presence of preservatives and other additives in oils that contain them.
AMSOIL states that a motor oil's grade label reveals its viscosity rating. The higher the rating, the more viscous the oil. These grades also indicate the oil's pour point, a measure of how the oil behaves in cold temperatures. Oils labeled with the prefix "W" before the viscosity number pour well in very cold environments, while other motor oils do not pour well in such conditions. Viscosity increases with temperature, but even cold engines require immediate lubrication.
According to California Scientific, the flash point of motor oil is the minimum temperature at which it ignites. This measurement is extremely important because motor oil is flammable. The higher the flash point, the more desirable the oil. Distillation removes many of the flammable compounds present in crude oil, and petroleum distillation removes more of them. Consequently, the flash point of quality motor oil is much higher than the flash point of both crude oil and petroleum.
Another important physical property of motor oil is its NOACK volatility score. This reflects its evaporation rate in extremely hot environments. Most applications require a NOACK volatility score below 15 percent.