Doug,
Excuse me if my next question makes me sound tribologically ignorant, but why if a 15W50 was good back in 1990 (per the Owner's Manual), is it not recommended any longer? Isn't viscosity, viscosity? If 0W40 and 5W40 (and one 5W50) is what Porsche now recommends, and nothing else, why don't they contact all Porsche owners (especially the non-forum browsing types) and let them know of these important changes to their Owner's Manual? Their TSBs are hard to come by for the average driver, so most owners would just continue to follow their Owner's Manual (unless of course they still go to their dealer for oil changes).
[Also, as far as I know, the car has been run on synthetic since early in it's life, and currently has a 20W50 synthetic in it. I have read/heard of air/oil cooled owners switching to the new 0W and 5W and leaks forming where there were none before...hence my hesitation to switch to anything lower than a 15W]
And thanks for everyone's feedback.