Originally Posted By: john757
Is 5w40 considered a 5wt oil or is it considered a 40wt oil? I understand that as the oil heats up it slowly goes down in numbers to about a 5 wt viscosity. But I have always heard the latter number was what the wt of the oil is considered, until I talked
to a spokesman on the Mobil website.
That's all backwards, it does NOT "go down in numbers to about a 5 wt. viscosity" at all. When its at operating temperature, its essentially the same thickness as a mono-grade SAE 40 would be at that same temperature. Where a 5w40 or 0w40 or 10w40 differs from an SAE 40 is what happens to it as it cools down to ~-30F. The lower the first number (before the "w"), the LESS the oil THICKENS as it cools.
So the easy layman's terms way to remember it is to flip your thinking around: The normal, natural state of an engine oil is not at room temperature, it is at engine operating temperature of about 212F and up. At 212F, all those oils that end in "w40" are required to fall within approximately the same viscosity range. Then, as you cool them off, that's where the number before the "w" comes in to play, and that's where they become physically different from each other. The lower the number before the "w", the LESS the oil CHANGES as you cool it to very low temperatures. But its not linear, so at, say, 20-30 degrees F (a cold winter morning in much of the US), a 5w40 and a 0w40 might or might not be basically the same. Odds are that the 0w will be thinner, but the difference only becomes really dramatic at very low, sub-zero temps.
