Oil on sparkplug
 

Recently I took my 2001 CLK 320 to the mechanic for a P306 fault code also known as a cylinder 6 misfire. At first, I thought it was going to be something simple like an ignition coil or ignition wires, injector or something like that, the car does have 116,000 miles on it, even the mechanic at first told me its likely a coil. however, it turned out to be something much worse. After looking more closely, the mechanic discovered oil on the spark plug. He told me he cleaned the oil off and put the sparkplug back in but it will happen again and he didn't fix the underlying issue of a leak. When I asked him what it was he just said that it could be a sign or faulty piston rings. *sigh* Great. Already matched the price a paid for the car last year by having the transmission rebuild and now I could have faulty piston rings. At this point, I paid $3000 for the car 1.5 years ago and have paid probably around an extra $5000- $6000 in maintenance up to this point and now potentially have a junk engine. I'm thinking this car is nothing more than a money pit at this point and a complete lemon.

Anyway, I guess for whatever reason the mechanic decided not to look into the problem and just said that it could be piston rings and said the plug will just probably have to be cleaned again in a couple of months or so. He also showed me pictures of the plug. There was light brown wet oil on the tip of the spark plug, not tons but enough to get it to misfire. From what I remember, there didn't appear to be much on the treads, maybe only the lower part. There was no sludge just wet oil and it only occurred in one cylinder. I don't believe the car has been burning oil, no burning smell, no blue smoke. I've since only driven the car home 9 miles from the mechanic shop with no problems but only time will tell. The mechanic just told me there's nothing to do at this point and to just clean off the spark plugs whenever they get oily again. That seems strange to me as I would think if there were bad seals, gaskets, or rings that it would leak again almost right away.

I should also note that this mechanic is a family friend that does really good work, is trustworthy, and has replaced my spark plugs, brakes, transmission, etc in the past. Also, I don't know about oil changes done by the previous owner but I've well kept up on oil changes and use Royal Purple full synthetic oil for high mileage engines. Any thoughts on the problem? Is it piston rings, valve cover gaskets, spark plug o-rings, etc? Is the engine just shot at this point? Thanks.

Normally when its oil leaking in spark plugs its valve covers. But you said light brown oil could be headgasket.

Misfire
 

I have the same car with a similar problem. Intermittent misfire P306 fault code. My mechanic checked plugs (They were old so we changed them but didn't notice any oil) and coil and they appeared to be OK. Did a static compression test and all was fine. Did a running compression test and failed only achieving about 60 PSI. He thought maybe bent valve or bad ring and examined inside the cylinder with a camera. No scoring on the cylinder wall or valve mark on piston. He checked valves and guides, cam and they appeared to be fine. He concluded the problem was mechanical but couldn't figure out exactly what and the fact that it appears to be intermittent is really strange. My best guess is weak valve springs. Anyone else have any other ideas??

The car is still running, the check engine light just came back on with a P306 once again after I accidentally took the engine up to redline. Still, haven't noticed any odd smoke on startup or under load. I think I'm just going to drive it until it dies.