He said the back of the intake valves are clean, that would leave out the intake gasket and anything entering the runner. I had severe
oil wicking through my rocker studs which cause the back of my intake valves to look like tar and my plenum had an
oil sheen as well as the runners, plugs were a mess also. I had to really do some cleaning and seal the studs. Problem went away.
Something doesn't sound right, any signs of a leak? What do the plugs look like? Pull the pcv and see if it's really puffing and what it looks like.