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The number in front of W is how cold it can withstand without losing viscosity. For example, 5w can withstand cold weather of 20-30 degrees Celsius. The engine oil does not lose viscosity. The number after W is the viscosity value when the engine is hot. And will the viscosity be lost only at 100 degrees? If so, cars whose engine oil temperature exceeds 100 degrees Celsius will be ruined.