My MB dealer uses 5W30 in my 1988 300CE. When I have my indy change it, he uses the same. I' don't know what brand oil they use. It is not synthetic.
FWIW, my car has 132,400 miles on it and burns less than a qt. of oil between changes. Valve stem seals were replaced at approx 75k and significantly reduced oil consumption. Still running with the original CAT.
I live in southern New Jersey. Life is good.

edit: I gave up changing my oil a looong time ago.