I run 0w40 FSS-spec Mobil 1 in the winter, and Mobil 1 15w50 in summer. The best indy shop in my area only runs 15w50 in all his customer's cars, and MB gladly filled mine with 0w40 in winter, and gladly used my 15w50 last time I went in, and the car is under Starmark warranty.

PS - mine is a 97 C230 as well.