People: for high temperature operation of an engine, you consider the high number in a multi-grade oil.
Thus in the 5W-30, it's the 30 weight that is significant. In the 0W-40, it's the 40 weight that is significant.
So the tech's contention that 5W30 is better at high temperatures than 0W40 is exactly wrong.
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For a SAE 5W-30 multi-grade oil, the first number (5W) is the 'cold' viscosity rating of the oil, the 'W' stands for winter, and the last number (30) is the 'hot' viscosity rating of the oil. Therefore, when the oil is cold (e.g. in sub-zero temperatures) it has a rating of '5W'. When the oil is hot (e.g. circulating in a hot engine) it has a rating of '30'.
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Which Oil Viscosity Grade Should I Use In My Vehicle?
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