Had my plenums off today to do the valve cover gaskets. First time I've had them off since I bought it. There was a layer of oil all over the inside of the upper and lower plenum and the intake manifold. I looked in the intake pipe and there was some at the very end where it attaches to the throttle bottle. Looked like maybe it was coming in from the vacuum line that attaches right there.

Any thoughts on what this could be or what things I should be looking at? I replaced the PCV valve today when I had the valve covers off but the old one didn't seem like it had failed.

Also, is there a product I can use to clean the the oil out of the intake without having to take it off the car?