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What Does the W Mean in Oil?

Oil being poured into a crankcase.

For you literalists out there, I know there’s no “W” in the word “oil.” But there is a “W” on every motor oil bottle (10W-30, 15W-40 and so on). So, what does the W mean in oil?

A Little History of OilNAPA Motor Oil 5W30 Synthetic Blend 5 qt (US)

The W should probably be a “V,” because it’s a measurement of the oil’s viscosity — its resistance to flow. More than 100 years ago, the Society of American Engineers (SAE) decided to set standards for pretty much all things automotive, including motor oils, and the viscosity rating is a result of these efforts.

Oil’s job is to lubricate — to reduce the friction of metal parts coming into contact with each other. But oils come in different thicknesses and weights (which is why you may hear people talk about “30 weight” oil, for instance), and they react differently to certain temperatures. An oil that’s ideal for an engine operating temperature of 200 degrees could be too thick to move through the engine on a cold winter morning. For the first few decades of the automobile, an oil change didn’t just mean fresh oil of the same type. You’d use a thinner oil in the winter, a thicker oil in the summer and maybe — depending on your local climate — something in-between for spring and fall.

Introduction of Multi-grade Oil

The first multi-grade oils were developed in the 1950s. They featured a chemical that allowed the oil to alter its viscosity based on temperature. The SAE came up with a number system for this. The first number, the one with the “W” next to it, indicates cold temperature viscosity —”W” for “winter.” The second number, after the dash, indicates viscosity at normal engine operating temperature. A lower “W” number means oil is less thick in the cold, and a lower second number means it’s thinner at engine operating temperatures.

Now that we’ve answered the question “What does the W mean in oil?” what’s the right oil for you?

The Right Oil for Your Engine

The simple answer might be “check your owner’s manual,” but it’s often more complicated than that. Some manufacturers recommend more than one viscosity, depending on the conditions in which you drive, so it helps to be realistic about weather, stop-and-go driving and other factors you live with.

By the way, no matter how thick it is, motor oil won’t act as a sealant. If you have an oil leak, there are leak-stopping additives you can use, but these will only work as a stopgap until a proper repair can be done.

Check out all the motor oil products available on NAPAonline, or trust one of our 17,000 NAPA Auto Care locations for routine maintenance and repairs. For more information on motor oil viscosity and the right motor oil for your vehicle, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Wikimedia Commons.

Mike Hagerty View All

Mike Hagerty is an automotive journalist whose work has been featured on radio, TV, in print and online since 1997. He's the Publisher and Editor of, and contributes car reviews to the Los Altos Town Crier and Previous outlets have included KFBK and in Sacramento, California, the ABC television affiliates and Hearst-Argyle and Emmis radio stations in Phoenix, Arizona; AAA magazines for Arizona, Oklahoma, Northwest Ohio, South Dakota and the Mountain West and

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