My 1987 240 with manual transmission is leaking oil around the filler cap. The engine has no other visible oil leaks. I checked the the flame trap and it was clear and clean. I checked the flame trap hose and the brass fitting on top of the intake manifold it, too, is clean and clear. I also checked the breather box by blowing into it with a hose and it sounds clear through the oil filler hole on the valve cover. Finally, I replaced the gasket and even added a cork gasket to that, but it still leaks. The leakage is mostly on the sides of the cap although I get some leakage around the pressure test port on the top of the cap. I would appreciate any suggestions on what might be causing this and what my next step in troubleshooting should be.

Greg