cglenn, I've researched this general subject for my Porsche Cayenne Turbo and decided Red Line engineoil is the best I can buy. As nearly as I can determine, RL uses the highest proportion of the best base oils available, and their additive package is as good as anyone's, overall.
Even in freezing weather*, I'd use RL 5W-40 year round, which is what I'm doing in very hot Phoenix. This 5W-40 is slightly thicker hot than RL's 10W-40, and of course it's thinner, slightly, when cool**. I believe that high-temperature film strength is very important for hard-working engines, and these 2 oils' HT/HS viscosities are about identical and quite high at 4.6 and 4.7, respectively. BTW, that's 12% higher than that of M1's 5W-40 Delvac 1/SUV oil.
If you have a conceptual aversion to viscosity-index improvers, use the 10W-40 in the summer (or year 'round if it never gets really cold where you live).
Myoilshop.com will deliver it for $84/case plus $7.80 shipping, and if you buy 2 cases, the price goes down 10%!
* but not if the temps are, say, around or below 0dF
** I can't quite bring myself to call 40dC (104dF) 'cold'.
[ March 13, 2004, 05:43 PM: Message edited by: Jeffrey Behr ]