I have had oil leaks from my 1995 E320 since I bought it 3 years ago. I bought it used with 35K miles (just under warrenty) and immediately took it in for service.
Hohen Benz of Carlsbad did a fine job of replacing the front oil cover, but unfortunately that wasn't the only problem. The main culprit was (and continues to be) the head gasket. And they also re-sealed the rear-end.
Since that time, every Feburary I visit my dealer to have them replace the head gasket. This year maked the third time in 3 years they have had to replace the head gasket for leaking oil. They also replaced the oil level sensor which was also contributing to the oil leak, they said.
This year I pressed the sevice guy about why it leaked so much, and he said "no-one knows why". He involved the shop foreman, who said the head checked out OK.
I have ready several messages on this (excellent) form, from E320 owners who have leak problems with the head gasket, as well as the rear-end. These problem appear to be typical for this model.
I would suggest you visit you local deal, and if you raise enough hell, you might get them to replace your head gasket, as they surely know this is a common problem with all E320 straigt-6 engines.