Did they replace valve guides?

There is a common error made by machine shops when pressing guides out and new ones back in. They fail to set the head on a stand that has the right angle and the drift used to push out guide slides over to one side and scores a groove down one side of the bore into which guide fits. If this occurs on an intake guide the vacuum in port will pull in oil. I would bet that if you carnked engine and ran it for only a few minutes and then pulled out plugs you could tell which cylinder(s) are the culprits. It can be fixed with metallized epoxy applied under the seat/washer that valve springs rest on in head.