As Mark said...this oil debate is turning into "politics and religion" debate. Generally the warmer the climate the heavier weight oil you can get away with using. That is why 0-30 works well in cold weather... My 91 300E with well over 100K miles seemed to do best on 20w50 conventional year-round. QS 15w40 is what I have been running in the 400E. The 500E has perhaps the cleanest valvetrain I have seen of any car with any reasonable mileage and it has had regular changes with conventional 15w40. Since it seems to be thriving on that, I will not "mess with success". Winter doesn't get much below freezing and summers are 95+ F her in Atlanta. On something of the M103 generation the 20w50 might serve better in summer, but for the closer tolerances of the M119's the 15w40 works just fine. If I were to spend the extra $$$ for synthetic I would likely run Royal Purple. I have a family member who runs that in his 93 500E and has been very happy with it.
Each to their own....Lee