I pulled the top cover to have a look at the plugs on my 99 SLK 230 because it had been using a bit of oil.To my surprise the trough had oil in it and the plug wells were nearly full of oil!! I think the only place the oil could have been getting in was around the O rings which fit over the plug well sleeve. I put newO rings in and added silicone. Hope this helps. Any thoughts, anyone???