What Is a Viscosity Rating?

A viscosity rating is a fluid's resistance to flow. The thicker the fluid, the higher the viscosity. Syrup has a high viscosity. Water has a low viscosity. Viscosity is oil's most significant property because oil provides an ultra-thin buffer between the moving parts preventing metal to metal wear. As oil gets hotter and thinner, it flows more freely because its viscosity decreases. As it gets colder and thicker, it flows less freely because its viscosity increases.

Motor oil viscosity is measured at zero degrees Fahrenheit and at 212 degrees Fahrenheit. If you are looking at a can of 10W-30 motor oil, the "10" is the viscosity measured at zero degrees with the "W" standing for winter, and the "30" is the viscosity measured at 212 degrees.

Motor oil additives are used to enhance performance. Additives called pour point depressants (PPDs) are added to improve protection at low temperatures, and additives called viscosity index improvers (VIIs) are chemical polymers that expand as the oil gets hotter.

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