10W40 Oil Facts

by Lexa W. Lee

A 10W40 oil is a multigrade oil that has two viscosity or flow grades. Prior to the development of multigrade oils, drivers had to use a thicker oil in summer and a thinner oil in winter.

Key Ingredient

Multigrade oils contain a polymer which slows the rate of thinning as oil temperature rises. The polymer also slows the rate of thickening as oil temperature falls.


A thinner oil will circulate faster through the engine on cold start. A thicker oil will protect better at high temperatures.

Meaning of 10W

W stands for winter. The oil must have a specified maximum viscosity at low temperature. The lower the W number, the thinner the oil. A 10W oil is thicker than a 5W oil in the winter.

Meaning of 40

The number 40 means that at 100 degrees Centigrade or 212 degrees Fahrenheit, the viscosity falls within given limits. At that temperature, a 40 weight oil remains thicker than a 30 oil.


Engine makers specify oils according to considerations like engine design and temperature conditions. Engine modifications may require a change in viscosity grade.

About the Author

Lexa W. Lee is a New Orleans-based writer with more than 20 years of experience. She has contributed to "Central Nervous System News" and the "Journal of Naturopathic Medicine," as well as several online publications. Lee holds a Bachelor of Science in biology from Reed College, a naturopathic medical degree from the National College of Naturopathic Medicine and served as a postdoctoral researcher in immunology.

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