If the road draft tube was plugged or the fresh air vent plugged I could see a motor puking out enough oil to soak the clutch causing it to slip. If they are not plugged adding a PCV valve to the system isn't going to stop or do anything about a big leak. I would be more inclined to think that the main bearings are at the point of excess clearance along with the rear main seal not being able to handle excess clearance. If it lost a plug in the rear of the engine you would have oil dripping after running for a 1/2 hour. Sometimes just sitting and idling won't really heat an engine up, they should be driven and worked to get things warmed up good. If it was mine I would get it warmed up by driving it and then look things over real good. It may be time to do a compression test, cylinder leak down test and maybe drop the pan and inspect the bearings and check some clearances.