It isn't just worn out gaskets and seals that cause leaks. I discovered with my XJ that if the CCV system is blocked or improperly set up, oil can can be pushed out the rear main seal, valve cover, the oil filler cap and/or the two elbow vents onthe front and back of the cover. If one or both of those CCV elbows on the valve cover are blocked, it creates a positive pressure situation in the system which pushes oil out wherever it can get out...

Make sure those hoses are breathing..