A multi-viscosity rated oil simply means it flows like the lower rating when cold, and lubricates like the higher rating when hot. Here's where things get a little complicated- - - -the oil does not change physical thickness as it heats up, so if someone is running a 10W-30, for instance, the oil stays the thickness of a 10 weight all the time. That's not enough "cushion" between moving parts in an engine with fairly large oil clearances. Parts get a running start and hammer each other into oblivion. I like to use at least 15W-40 oil, and straight 20 or 30 weight is better unless the engine has been rebuilt to very close tolerances. The "W", by the way, does not represent "weight"- - - -it stands for "winter", denoting the temperature at which the cold flow rate is tested.
Jerry
