operating temperature. The oil thickens as it cools from this temperature. The low number is an indication of how much the oil thickens from this high number thickness. An ideal oil would not thicken at all, so the ideal oil would be something like -100W40. I expect numbers lower than 0 to come out soon. (you pick your high number, that's a different discussion) Bob is the oil Guy has some good explanations of this stuff.