Using a heavier weight oil will just start causing hydraulic lifter problems next. You don't need to see smoke out the exhaust to be burning more oil. 120,000km is enough mileage to have this problem if the oil and filter were not changed every 3,000 miles or every 5,000 miles when using synthetic oil. An older motor can use half a quart of oil every 1,000 miles. Make sure the PCV valve is not plugged up. I wouldn't put any kind of sealer in the oil.
Mitsubishi Master Tech