Bad valve stem seals are 90% the cause of most oil consumption and exhaust smoke in Mitsubishis. They usually go bad @ about the 200-250,000 km mark or 10 years if the engine hasn't been religiously maintained. Mixing synthetic and mineral oils should also be avoided at all costs as this will gum up a good engine in no time. If you need to do the valve stem seals I'd advise taking the head off and giving it a once over and replacing the timing belts. Removing the head will give you a chance to properly assess the condition of the valve stems and coolant galleries etc. Yeah it's not cheap getting a good quality head gasket set and maybe sending the head through a machine shop for valve seal cutting and pressure testing but once it's done it'll be rock solid for another 10 years.