Perhaps it's the valve cover gasket.?. Around those cam humps, there are 4 places where the gasket has 90 degree angles in it.
IIRC, those are the sites where sealer is applied to insure no leaks.
I'm not sure, but I think there is a certain order to tighten the cover bolts to insure a good seal.
Was the seal replaced when you did the valve adjustments?