i have phantom moments when traction control suddenly turns off and engine light comes on
At temps below operating temp, any oil whether it is 0w, 5w or 15w is too thick. Doesn't matter whether your climate is 5C or 40C, 0w is better until the engine heats up.
Sure in a fairly hot climate like Oz, 15w is not as bad as it would be in places where you start cars from 0C fairly regularly.
So clearly you want the first number to be as low as possible.
Now for the second number, Mitsi themselves say that 30 weight oil is ok in climates up to 35C and 40 weight is ok above that. I have multiple owners manuals showing this. The job of the thermostat is to keep the water temp inside a very small range and in turn keeps the oil temperature in a very small range as well. The engine is designed to accept a certain amount of oil flow, and that flow is acheived normally by a 30 weight oil's viscosity and it's ability to flow a certain amount at that temperature. I think Mitsi suggest a 40 weight oil for hot climates not so much for normal operating conditions, but for extreme situations where a cheap 30 weight would break down rather quickly.
If you run 15w50, you are a fool. Simple as that. You don't understand enough about oils to make that decision yourself, so listen to myself and Mitsi and put in the correct oil.