oil in air filter housing

I was given a 1983 oldsmobile cutlass broughm with 27,000 original miles. It has a v6 gasoline engine.It would stall at idle but runs like a champ on the open road. I had check engine light come on so I took it to the local mechanic. He did not fix the problem and informed me of oil deposits in my air filter housing. I suspected a vacum leak but he said he did not find the cause with a diagnostic check. He offered a tuneup but would not gaurentee this would fix the problem. When I drove it to the shop it did not stall out as before, just idled a little rough. What would cause oil to get into the air cleaner area with only 27,000 miles as I feel the oil into this area is causing the rough idle. Please reply to judyedy@ juno.com
Thank you, Ed