Hard to say what the issue is, some things to look for based upon my 560SEC experience are: valve covers, heads, rear main seal. All three of these can cause what you are experiencing. Last two are expensive. A good visual inspection will help you. You should be able to see around the valve covers and heads with a good light- if heads only, you won't see oil on surfaces above the head gasket. This assumes a clean engine to start with. One problem is that all of them could be leaking a bit, which makes finding the source difficult.
My mechanic used a dye in the oil that shows up under ultraviolet light (I think) and with this we were able to see very clearly leakage from the head gasket. I had an upper engine rebuild and now the upper block is dry, dry dry. I still have a small leak from the rear main seal, and am watching closely but don't want to repair until I need transmission work. The leak is small today, but a bit annoying, as occasionally a few drops of oil drips onto the hot exhaust and causes smoke. Not every day, but still annoying.
Good Luck!

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles