Most common reason is crankcase blowby. On flathead engines, a blown head gasket won't cause any oil inhalation.Only 4 reasons why oil ends up in the filter
1) mower tipped on it's side
2) oil over filled
3) blown head gasket
4) failed breather valve
Not meaning to be arguementative but on a 90000 series engine people tipping the mower carb side down or back wheels up would account for better than 90% of oil in the filter.Most common reason is crankcase blowby. On flathead engines, a blown head gasket won't cause any oil inhalation.
Here's another weird reason: people deliberately oiling the filter. I've actually seen this recommended in an old (~1980 vintage) book. I just cleaned up a filter that was soaked with oil from the top; i.e., it could not have been introduced by any flaw other than ignorance. Several days ago when I was a young man, I actually tried this myself. Much amusement was had by everyone but me.
Sir, I have never attended a motor which did not already have excessive wear in the bore. I count myself lucky if the crankshaft is not bent.Not meaning to be arguementative but on a 90000 series engine people tipping the mower carb side down or back wheels up would account for better than 90% of oil in the filter.
Blowby is far less common because it will either be caused by excessive wear in the bore or excessive oil in the sump.
However most 90000 series will have a tank mounted carb which precludes fuel draining into the sump over filling it and causing eith blow by but most usually oil pumping out the breather tube into the filter.
I am also seeing that issue with the new Briggs plastic carb engines, the Honda GCV engines, and the MTD made in China engines that are being used on push mowers and pressure washers. Tipping any of them carb side down will almost guarantee oil in the air filter, and two if not all three of the engine series requires replacement of the cartridge style air filter.Not meaning to be arguementative but on a 90000 series engine people tipping the mower carb side down or back wheels up would account for better than 90% of oil in the filter.
Blowby is far less common because it will either be caused by excessive wear in the bore or excessive oil in the sump.
However most 90000 series will have a tank mounted carb which precludes fuel draining into the sump over filling it and causing eith blow by but most usually oil pumping out the breather tube into the filter.