My 98 E320 has a minor oil leak from the bottom of the oil filler neck (the black plastic neck connecting the oil filter cap to the valve cover).
At first I thought it was the oil filter cap, after replacing the oil filter cap I noticed it was leaking from between the bottom of the oil filler neck and the valve cover.
I replaced the gasket which helped alittle, my next step is to replace the oil filler neck.
Since the leak is very minor, I have been wiping the oil with a shop towel every few weeks.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles