Metricman - thanks for the info. MB made the 300-SEL from 1988 -> 1991. According to a couple of other sources, the valve seal/guide problem existed on '88-'89 models. 1990-1991 models supposedly came with new/improved guides/seals. Mine is a '91 model. The plugs have a tannish color after 10,000 miles of driving...no indication at all of oil getting past the guides/seals/piston rings/anything of that nature. When I change oil, I pull the large plastic encapsulation cover from the bottom. There's a spot on it where oil is landing that's pretty much underneath the distributor area. Regards.

Mike Murrell
'91 300-SEL
Boerne Texas