I have 2 Porsches (yes I have a Ferrari - but it seems there are better experts here than I can get to on Porsche sites I've tried). I just had oil changes done on a 1982 911 SC and a 1995 993. The shop used Mobil 1 0-40 for both cars, yet the 911 manual suggests petroleum based 20-50, and the 993 manual lists 15-40/15-50 synthetic. So the questions are; - for the 911 is synthetic oil inappropriate (can loosen up deposits ...) and is the lighter weight an issue (the car does not get driven or started in Winter - Minnesota!) - For the 993, is the lighter weight oil a problem for summer driving? Thanks, "Luca"