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Historically, the first number associated with the W again 'W' is for Winter, not Weight is not rated at any single temperature. The 10W means that this oil can be pumped by your engine as well as a singlegrade SAE 10 oil can be pumped. 5W can be pumped at a lower temperature than 10W. 0W can be pumped at a lower temperature than 5W, and thins less at temperatures above 99 °C 210 °F. The second number, 30, means that the viscosity of this multigrade oil at 100 °C 212 °F operating temperature corresponds to the viscosity of a singlegrade 30 oil at same temperature. The governing SAE standard is called SAE J300. This classic method of defining the w rating has since been replaced with a more technical test where a cold crank simulator is used at increasingly lowered temps. A 0w oil is tested at 30f, a 5w at 25f and a 10w is tested at 20f.
The realworld ability of an oil to crank in the cold is diminished soon after put into service. The motor oil grade and viscosity to be used in a given vehicle is specified by the manufacturer of the vehicle, but can vary from country to country when climatic or mpg constraints come into play.Many new vehicles are marked to use 5W20 oil Honda, Ford, and more recently Toyota which is not much thinner than a 30 weight oil. Naysayers of 20 weight oil's ability to protect engines should note that typically, 30 weight oils shear down into the 20 weight range anyway. Most engine wear is during startup and warmup period, where the thinner 20 weight oil's flow is desirable. Overall, lab test results of the wear metals contained in used oil samples show low or lower wear with 20 weight than 30 in applications it is specified for.