There is something up with #1 piston and/or combustion chamber. This could mean bad rings, bore damage to #1 cylinder, a crack in the combustion chamber - or an easier fix like valve stem seals. Most Mitsu oil consumption issues revolve around bad valve stem seals. Compression check to tick the rings and bores off the list. Valve stem seals have a life of about 10 years before wear and tear or age takes them out. If the engine has been regularly serviced and had good quality oil and/or additives used every service, it can extend the life of the seals (and pretty much everything else...)