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Explaining viscosity designations

“What does 10W-40, 5W-30 etc. mean? I would really appreciate it if you could explain it in simple language so I can teach my students. There are so many different grades of oils. Which one is better to use in summer than winter (Chicago) and vice versa? Thanks a lot.”

 

Vijay Patel, Lake Zurich, Illinois

Answer

In simple language, the first part of the viscosity designation (W grade) is an indication of the product's ability to help an engine crank and start and for the engine to pump the lubricant. The lower the number (0W is the lowest), the lower the temperature the product can be used. So the W grade is related to the lowest temperature your engine sees when you start the engine on the coldest morning of the year. But also keep in mind that a lower W grade pumps and helps an engine to start better than a higher W grade. The second part of the viscosity grade is related to the viscosity your engine sees at operating temperature. In this case, a higher number is a higher viscosity grade and provides more viscous oil at operating temperature than lower viscosity grade oil. It is not safe to assume that a higher viscosity oil is always better for your engine because other factors, such as engine design, fuel economy and power, are also related to operating viscosity. You should always consult your owner's manual for the right oil to use for your particular engine. For more information on the viscosity grade specifications, a quick search of the Internet will identify several websites where information about SAE J300 (Viscosity Properties Test) can be found.

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