I have the same problem with my '80 635, only both the chrome and the paint acquire discoloration. The chrome acquires a black tint, while the grey bumper acquires a yellow film. My car does not burn oil either, but the exhaust is more "potent" than normal due to the car's 20-year old cat. I haven't seen this problem with other cars becuase most cars have their exhaust outlets underneath the car or the rear bumper. With the Euro 6'er, exhaust gases are free to flow right over the rear valence. Waxing the bumper AND chrome at frequent intervals makes residue removal easier.