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Autotrade: Oils and Lubrication - what does the 'w' mean in relation to oil?

Grades with the letter ‘w’ are intended for use at lower temperatures and are based on a maximum low temperature viscosity and a maximum borderline pumping temperature, as well as a minimum viscosity at 100ºC.

Figures such as 10w-40 relate to the thickness during operating. 

The first number denotes its viscosity when the vehicle is cold and the second when it is at operating temperature, normally about 100ºC.

As a rule 10w oil can operate at temperatures as low as -25 degrees centigrade.

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