An engine Oil leak may develop from the PCV vent housing on the valve cover and/or the inspection plate on the front of the engine.Google+
Car Problem Reports
Mercedes-Benz S500 Oil May Leak From PCV Vent Housings and Inspection Plates
Mercedes-Benz S500 Problem
Average Mileage: 97,500 mi (90,000 mi - 105,000 mi)
This is a common and easy repair. My right side one is "weeping" and has taken about 2 years to get to the point it needs to be removed and resealed. There is not a gasket, they use RTV sealant.
Visitor, 2000 Mercedes-Benz S500, 105,000 mi
Massive oil leak, pulled away from a parking spot and there was a pool of oil, it had been leaking in the front of the engine, could not tell what it was, managed to get it back home by pouring into it every mile or so, just the small inspection on the front right of the engine (facing the engine), the seal had failed, I just put cardboard in and the leak stopped. This has got to be quite prevalent on these cars because the seal was not broken, just worn and shrunken, causing this massive leak.