The smell is so nasty, when I drive the truck with the heat on, the smell will come and go, it smells especially if you put your nose in the front wheel well, I know that valve cover gasket if usually whats bad if you smell oil while driving, not sure though since the plugs didnt have any oil deposits on them??

 

Take a flashlight and look for leaks around the valve cover. Even if you see no leaks, make sure all the bolts are tight on the cover. A few of mine were hand tight when I checked them

 

look for a slight discoloration on the exhaust manifolds. this is where the oil would be seeping down and burning off.

retorque your valve cover gaskets. they have likely loosened up to finger tight or less. if that doesn't work, and you are sure about the leak, then replace them with the super nice felpro rubber gasket. shop smart from rockauto or amazon as they are expensive.