Excerpt from AMSOIL site:
To better understand, in a 5W-30 oil, the "W" stands for winter performance. In winter weather the 0W oil will flow like a 0W oil, and the 5W will flow like a 5W oil and a 10W will flow like a 10W oil just until the engine warms up. This multi-grade performance allows faster oil flow during cold start, however when the oil warms up, they all reach the designated 30 weight viscosity.
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