I don't know if this is true or not but the way it was explained to me many years ago was that the lower number is actually what the real weight of the oil is and the higher weight is accomplished by the additive package and isn't really a real weight you can bank on under extreme conditions. That's why I put so much "weight" (I'm so funny) on the lower number.
The way it was explained to me is that they still don't know how to make a thick oil behave like a thin oil but they can make a thin oil behave like a thick oil when it needs to, at least for a little while.
I do know that the "W" stands for what the "weight" of the oil is in "Winter" conditions but again, my understanding is that this is also what the oil started out as before they pumped it up on additive "steroids". In fact, I was told that in many cases, a car is better off with straight 30 or straight 40 weight for this reason as long as said car is not started in cold temperatures.
The bottom line is I want a fairly thick oil in my pan because I live in a hot climate, M119s are hot running cars, and I also believe higher mileage cars need a heavier oil to fill the looser clearances of their higher mileage engines. On the other hand, I know I don't want to go too thick though because too thick of an oil robs too much power just for the pump to circulate it through the engine.
We've been talking about this and other oil issues here:
http://www.500ecstasy.com/forums/showthread.php?t=6209
Regards, Eric
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