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Oil Numbers Decoded

10W-30, 5W-30, 0W-20, and the list goes on! What’s with all the numbers and what do they mean? What makes one oil different from another?


It’s all about viscosity. Simply put, viscosity is how well a liquid sticks to and coats an object. For example, water has a low viscosity - it doesn’t easily keep a coat on a surface. Now consider honey. Honey has a high viscosity. It will create a thick coat on your spoon or measuring cup and it doesn’t easily, quickly, or cleanly pour off a surface.


The first number listed for an oil grade is the viscosity rating at cold temperatures. The higher the number, the thicker it is at winter temperature. The first number is grouped with the “W” to indicate winter. 


The second number indicates the oil viscosity at the vehicle’s normal operating temperature, which can be around 200℉. Due to the additives in modern oil, it actually becomes more viscous at higher temperatures and is able to better coat the internal mechanics of an engine. 

The discussion of engine oil can be a very complicated subject. If you would like further information, please visit the Department of Energy’s Website.  


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