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Understanding Oil Viscosity

The reality is that the oil you choose can make a significant difference in engine performance and wear over time. Here’s how to decode your oil type and why it matters. 


For the purposes of this article, we’ll focus on multigrade oils, which are the oils most commonly used in cars and trucks. These oils are identified by a number, followed by a “W” and a dash followed by another number. For example, 5W-30 or 10W-30. These are oil grades, established through a system developed by the Society of Automotive Engineers (SAE).  

An oil’s grade describes its viscosity, or thickness, at different temperatures. For a clear example of viscosity differences, think about honey, which is very viscous, compared with water, which has very little viscosity. 

The first number in an oil grade describes its cold-weather viscosity (the W stands for winter). The higher the number, the more viscous your oil will be at colder temperatures. So a 5W-30 will be thinner and flow better when cold than a 10W-30. If you live in a Northern climate, you might have experienced the cold-start properties of oil first hand. Next to battery depletion, oil that is thicker and difficult to move when cold is a primary contributor to hard winter starts.  

Now let’s look at the second number. The larger number after the dash on an oil container indicates the oil’s viscosity at hot temperatures. So to use the same grades as above, the viscosity when hot would be the same for both 5W-30 and 10W-30 oil.


Vehicle manufacturers develop their engines to operate optimally using specific oil viscosities, so it’s important to follow the recommendations in your owner’s manual. 

An oil that is too thick can put too heavy of a load on your engine, worsening fuel economy and ultimately shortening the engine’s life. If your oil is too thin, parts can wear more quickly, again shortening engine life.