I would never use a 15W-xx oil in cold climates, to take Alaska for example, a 15W-xx oil would be like thick honey in that climate and would take forever to get circulating inside the engine providing it even starts due to the heavy drag from the oil pump trying to force the thick oil through the engine, hence why an engine turns over slower in cold climates. Whereas a 5W-xx or even a 0W-xx oil would be a lot easier to get circulating through the engine on a cold start in cold climates.
As the original poster already admitted that the car never really gets warm, that is the worst thing for an engine, oil really doesn't have good lubricating qualities until it is warmed up, but "warmed up" does not mean the engine coolant is at or close to operating temperature either, the only real way to tell is with an oil pressure gauge, the engine oil is considered warmed up when the gauge lowers closer to it's normal pressure reading when the engine is fully warmed and at idle. Running an engine on short trips and never fully warming it up is considered "severe duty" service and dictates more frequent oil changes than an engine that sees a lot of highway usage. Sludge buildup is also more common in engines that run a lot of short trips compared to engines that see a lot of highway use.