Pull the spark plug and see if it is oiling up, might be a valve guide seal, mine did exactly this. Does it puff out a small amount of blue smoke after it has idled for a few minutes ? Just a thought . . .

__________________
1993 190E 2.0L (Euro.) sold
1991. 500 SEL (Euro.)
1991. 300 CE (Euro.) sold
1993. 500 E (U.S. spec.)