Those engines do burn oil. Clogged PCV would be the first to eliminate. Go w/a HD 5w-40 like Rotella. You can try a piston soak, a mix of 2-3oz mixed light oils and solvent, poured into sparkplug holes, crank, sit overnight, crank... Do an intake Seafoam cleaning with the old oil in, right before draining it.
I use 2-cycle oil in my gas at 500:1 (3-4oz/10G). Use double for the first treatment. I actually mix some Techron in there too.