Based on my two short years of experience... I haven't had any problems running Rotella 5-50 (er, or whatever the hell it is- thanks Tower) synthetic during the summer. Temps often in the 90s with commuting traffic.
It appears the stuff is gold- I've seen in recommended for use in Subaru WRXs as well for many of the same reasons mentioned here.
But then, I won't be able to give a definitive answer for another 10 or 15 years.
spwg