My mower has begun pump oil out of the oil filler tube even though the cap is in place. I have cleaned the air breather filter as specified in the users manual. It is a poly foam filter that is cleaned with warm soapy water and lubricated with oil after washing and drying. This has been a very good mower since i purchased it new in 1986 or 87. Can anyone tell me what might be the problem causing the engine oil to be expelled from the engine?