I owned a landscaping biz for 7 years and have changed more oil on aircooled mowers than bullets were fired in WWII, ok maybe not but I digress. It is very common on small engines (under 8HP) to have the gray and milky used oil come out. On the larger engines it rarely did this. My only guess is that the larger engines were commercial quality and were built for the long haul where as the smaller engines have very loose tolerances. Of course I only went through a couple of push mowers in 7 years and they were the cheap versions so I would say that if you change the oil once a year and check/change the air filter, the engine will last 10 years easily.