My 2000 Jeep Cherokee (that I bought about a week ago) with a 4.0 L Straight Six is leaking oil around the valve covers. Oil leaks from there and drips onto the manifold and burns. The leak isn't major, it appears to be a small leak at the moment. I tightened the valve cover bolts this morning but it still leaks. I'm guessing that its leaking from the valve cover gasket or the header gasket. Could this be anything else? Advice on how to repair this would be greatly appreciated! 