Originally Posted By: Stang40
Thanks Kuato, that's what I was thinking, but it almost made it look like a 0w40 was thicker at start up than a 5w30, which doesn't make since.
Well, there IS a temperature at which the 40 weight becomes thicker than the 30 weight oil, and it will remain so as the temperature climbs.
Someone referenced a site a few months back (couldn't find it) where you could plug in oil weights and get an estimate of viscosity, based on 3 data points (low temp viscosity, 40C and 100C). I compared a 5w20 to a 0w30, and the 0w30 was thinner until about the freezing mark, after which it became thicker and stayed that way all the way up the chart. I'd imagine your comparison would be similar, but the oil temperature where one oil becomes thicker might be somewhat different.
The thing is that the right oil will act as the correct viscosity at a given temperature, and once operating temp (100C) is reached it will be the weight after the "w". I've moved exclusively to 0wxx oils year round, with Montana winters getting down to -35 at times it is IMO pretty much a requirement.