Intake Manifold Carbon Build Up?

[FWB]

that is where your oil residue is coming from. is there a baffle where you pull the pcv from the crankcase? if it's from the valve cover you might be sucking raw oil into the plenum.

there isn't a trick to cleaning it, because if its that way in the runners the backs of the valves prolly look worse. when the oil mist hits the hot valve , it immediately crystallizes on the back of the valve. i would pull off the intake to clean it, and then it will give you the ability to see what the intake valve looks like