The crankcase breather (right crankshaft end cover) relies on a metal sleeve or collar rotating in an oilseal.
If the seal is torn or the sleeve is worn (a poem!! :-D ) the engine can pump oil past the breather into the airbox. As you lean the bike onto the sidestand almost all the oil drains out through the air-bleed pipes onto the left chrankcase cover leaving about a thimble full behind.
You can check this by disconnecting the breather pipe as it leaves the engine. I got the shop to repair mine under guarantee and they took half an hour But someone on this site has done it themselves.
Good luck Jon G