There's a long discussion of this on another thread, but it seems pretty common for the 300 SEL. Oil blows up through the valves and dribbles down the block, mostly over the #1 cylinder. The #1 plug will foul early and often. I'm betting that getting the valve seals replaced will drastically reduce my oil consumption (quart/750 miles, no smoke), keep my idle smooth, and prevent my having to replace the #1 plug 5 times as often as the rest of the set.

Tim
'89 300 SEL, like one of my kids