Hey Harry:
Thanks for the information with regard to Mobil 1 syn! I did not know that the ZDDPlevels are not acceptable. That is NOT good.
As far as the chart from the owner's manual, that looks just like my owner's manual for the 1983 MY. In fact, that is why I chose to run in the Winter with a different grade. I do seem to get better performance out of the 15W-50 most of the year.
This year, here in Eastern Washington, we have had dry days with very, very cold nights and low temps for the day on average. So, it seemed like the best decision for my daily driver 944, but certainly may not be right for the 911. I have been driving the 911 to keep the battery charged and burn some carbon, but only this year since there is no salt or snow on the roads. I never drive it otherwise in Winter months.
Appreciate the info. Time for me to read up on the additives too.
Once again, "Thanks!"