i removed a spark plug from 1994 E320 and it had oil all over it. The car was running fine, no indication of engine misfiing. I had a red engine light come on after an oxygen sensor fixed it the first instance. i washed the engine and then it came back on a couple of days later. A post said water gets in spark plugs and stays there for some reason causing engine light. I was checking this when i noted all of this (fresh) oil dripping off the plug when I pulled it out.Bosch platinum +4 R3. I am going to get new Bosch copper plugs. Still do not know what is best recommendation for plugs.

I hope none of the other plug holes have oil.

Might just need a new valve cover gasket set including the donuts around each spark plug hole.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1994-Mercedes--benz-E3--0--Sedan-Engine--Mechanical&yearid=1994%40%401994&makeid=63%40%40MERCEDES%2DBENZ%40%4063%40%40MERCEDES%2DBENZ%40%40X&modelid=6391%40%40E320+Sedan&catid=240154%40%40Engine+Mechanical&subcatid=240451@@Valve+Cover+Gasket+Set&mode=PA

Is replacing the valve cover gasket an engine tear down? How big a job is this ?

No, not a big job, but if your wiring harness is original you'll probably ruin it.

http://mb.auto.pl/wis/w124/CD01/Engine/104/01-0500.pdf