I have '61 172B with an O-300D that is leaking oil from the oil filter adapter.  The leak is from a small vent hole in a plug.  My A&P has contacted Cessna and learned the there were two differnt plugs used on that adapter one without a vent hole and the one with the vent hole. So far Cessna has not been able to explain the change or if the vent hole could by welded.  Has any body ran into this problem before?