If you are not sucking air through the system as designed, you can have oily air build up inside the can. Keeping those gaskets in place is necessary to keep air flowing properly.

Also check the oil vapor separator for blockage. There is a hose running from the top of the valve cover to the top of the air cleaner box. That part inside the air cleaner can is the separator and where the vapor is (theoretically) separated into oil and air. The oil goes down and out to the crankcase, while the air goes to the intake.

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1982 300CD Petrol/Black Leather