A little more info here. I have examined the air filter closely and believe it is very much plugged. It may not look so bad in the pics but with a magnifying glass, not so good. The surrounding gasket was going brittle as well. You have to consider it has 72,000 miles or more on it, so it's done.
I found the vent line, from valve cover to intake plenum, draws clean air from the plenum into the engine while running [idle]. The engine does not have a PCV valve. There is a small passage in the head connecting the interior of the engine to the intake runners for cylinders 2 and 3. It appears under low throttle air flows from the clean side of the air filter thru the engine and to parts of the intake manifold [ I have verified this]. When considerable throttle is applied things change. Manifold pressure would drop and a partial vacuum could be applied to the PVC vent line [plenum to valve cover].
If the air filter were plugged, I would assume considerable vacuum could be applied to the engine internals and maybe draw some oil into plenum upstream of the throttle body. For this to happen I would think there would have to be a fair amount of blow by, but I don't really know. Time will tell as I have replaced the air filter and I will monitor the oil level and look for gunk on the walls of the throttle body.
06 GM 2.2 ecotec naturally aspirated engine