Verified answer

You said oil cleaner and I think you mean air cleaner. What causes that is, there is too much pressure in the crankcase that has to escape somewhere. You have a PCV valve that works to relieve that pressure and it vents into the air cleaner. There are a few things that causes oil to accumulate but the most common is, your rings are worn and compression gases are escaping past the pistons into the crankcase causing to much pressure for the PCV valve to efficiently handle and forcing oil to escape through the PCV system into the air cleaner. You can check this by removing the oil filler cap while the engine is idling. Notice whether there seems to be some "smoke" escaping and possibly blowing some oil droplets with it.

Like I said, there are a few things that can cause that, but this is the most common.

Bad compression and/or exhaust valves. Oil in gas. To much additive? Like Marvel.

stuck or broken piston rings