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0w is a low-temperature pumpability rating, not a viscosity rating.
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That's an oversimplification. There are two tests that an oil must pass to get a given w-rating; Cold Cranking Simulator and Pumpability. The SAE J300 requirements list maximum allowable viscosities for each, but at different temperatures depending on the w-number being tested for.
0w = 6200mPa*s @ -35C for CCS, and 60,000mPa*s @ -40C for Pumpability
5w = 6600 @ -30C, and 60,000 @-35C
10w = 7000 @ -25C, and 60,000 @ -30C
15w = 7000 @ -20C, and 60,000 @ -25C
20w = 9500 @ -15C, and 60,000 @ -20C
25w = 13,000 @ -10C, and 60,000 @ -15C
The lower the viscosity, the easier the oil is to pump at winter temperatures, and the lower the stress on the oil pump drive, starter, and battery. Notice the big jump in allowable viscosity between 15w and 20w; 7000 to 9500. I would think twice about putting a 20w oil in my engine if temperatures much below freezing are expected.