What Does 10W40 Stand for on Oil Containers?

Written by stephen byrne
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What Does 10W40 Stand for on Oil Containers?
(S. Byrne)

When it's time for an oil change (every 3,000 miles or so), you must determine which oil to use. There are as many different types of motor oil as there are cars on the road. By understanding what 10W40 actually means, you can make an educated decision when it comes to choosing the right motor oil for your vehicle.


In 1905, the Society of Automobile Engineers was established, with Andrew Riker serving as president, and Henry Ford as the society's first vice president. This Society remains a powerful force in the auto industry, and is represented by the SAE seal found on all certified motor oils. The SAE developed standards for motor oil that graded it according to its viscosity.

What the Numbers Mean

The "W" stands for winter, and the number before the W represents the oil's viscosity at zero degrees F. The number after the W represents the oil's viscosity at 100 degrees C, which is the upper temperature range of a typical engine.


In the simplest terms, viscosity is the measure of how easily a liquid pours. Temperature has a direct effect on oil viscosity; higher temperatures reduce the viscosity of oil, reducing its strength and lubricating properties. Conversely, freezing temperatures can make high-viscosity oil stiff and thick, which can make it difficult to turn the motor over.

Multi-Viscosity Oil

Today's motor oil is engineered with properties that allow it to retain its lubricating properties at high temperatures, yet remain fluid at low temperatures. 10W40 will protect your engine in weather exceeding 37.8 degrees C, and it will be thin enough to start your car even when it's minus -6.67 degrees C.

Check the Owner's Manual

Although you know what the numbers represent, you should still read your owner's manual to determine the correct oil for your vehicle. Auto manufacturers recommend a range of oil ratings for each of their vehicles, depending on the operating environment.

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