How Important Is Viscosity in Engine Oil?

Viscosity or kinematic viscosity--the resistance of a liquid to flow--is one of the most important characteristics of engine oil. The viscosity of engine oil must be sufficient to lubricate the moving parts of an engine at normal operating speeds and temperatures.

  1. Function

    • The function of engine oil is to reduce friction and wear and tear on engine components, particularly engine pistons. Too thin oil can cause permanent leaks in the engine seals, while too thick oil can lead to high operating temperatures and difficulty starting an engine in cold weather.


    • Viscosity is the result of internal, molecular friction within engine oil. The greater the friction the greater the viscosity and the greater the load the oil can withstand.


    • Kinematic viscosity of engine oil is commonly measured in centistockes (cSt) or Saybolt Universal Seconds (SUS) at a given temperature. In this case, the higher the number the higher the viscosity or thickness of the oil.

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