Id oil consumption due to sticky rings is your issue, try a piston soak with either seafoam or B12 Chemtool. Had a mower that would burn quite a bit of oil, did a piston soak and added some seafoam to the oil and ran it for 20 minutes, repeated the soak and changed the oil (Rotella). Mower quit burning oil entirely and the oil now stays clean, going on it's third season on this oil and it looks like the day I poured it in. Probably only put 20hrs a year on the mower but I was ready to buy a new one over the oil burning, mower's an almost 30yr old Toro and it runs like new.