Recently had a similar issue: minor leak from valve cover gasket creating fumes. Found out that one quick fix would consist of simply tightening the nuts holding the cover. Somehow some of them became loose over the years creating this very minor leak. Either way replaced only one gasket and decided not to replace any other gasket upon inspecting the gaskets and noticing that they were in "excellent" shape. This to prevent complications as it happened here. Good luck, probably one of those may have dirt stuck creating a minor gap or so...