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 CLK500 Oil May Leak From PCV Vent Housings and Inspection Plates
Mercedes-Benz CLK500 Problem
Visitor, 2005 Mercedes-Benz CLK500, 42,000 mi
Left valve cover leaking oil near oil filler cap. Removed smaller cover from main valve cover and used blue form a gasket to seal the leak.