What Is Oil Viscosity?

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Oil is rated by the Society of Automotive Engineers (SAE) for viscosity. Viscosity is a rating which describes the thickness of a fluid. Engines are designed to use oil rated at a certain viscosity. Viscosity is very important in preserving engine life and reducing wear-and-tear.


  • To determine viscosity a measured amount of oil is heated to 100 degrees Celsius (210 degrees Fahrenheit) and poured through a device called a viscometer. The time it takes the oil to flow through the viscometer determines the weight of the oil.

Oil Weight

  • Oil is often referred to in terms of weight. Oil weight is not to a measurement of ounces or pounds but viscosity. The terms weight and viscosity are interchangeable.

High vs. Low

  • High-viscosity oil will have a higher number and consequently take longer to flow out of a viscometer. Fifty-weight oil is thicker and will flow out of the viscometer more slowly than 20-weight oil.


  • Modern oils sometimes have a multi-weight rating. Oil rated at 5W30 would be an example of multi-weight oil. Multi-viscosity oil is tested at colder temperatures as well as higher temperatures so it performs like 5-weight oil at colder temperatures and 30-weight oil at higher temperatures.

Why Multi-viscosity

  • When an engine is cold, 30-weight oil will flow slowly and not circulate quickly enough to limit damage to the engine. Having multi-viscosity oil in your engine will allow the oil to be thin enough to circulate readily in a cold engine yet retain its needed thickness when the engine warms.



  • Photo Credit old engine image by John Sfondilias from Fotolia.com

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