From the Mitsubishi Workshop manual;
** SAE 5W-30 may be used for operation in very
cold weather areas where the lowest atmospheric
temperature is below -23°C (-10°F).
I have been working on these engines for quite a few years now and have tried so many oils just to find one that is good.
Mitsu actually recommended a 10/40 oil when the engine was developed.
We found that the best results were given from using Pro-S 10/50. That is based on engine wear, oil pressure, engine component reaction [cam follower tick is almost eradicated].
I have built numerous GTO engines in various spec and using various components and since finding this oil, have used no other. We use it as a general service oil and as an oil on upgraded engines.
There are other oils out there that i haven't tested but mainly because i don't really need to, this stuff works, not only for me but for a few hundred other GTO owners in this country.
I'm not saying it's the only oil for your car, you must make that decision but just be aware that the majority of Mitsu franchises don't even know how to take these engines to bits, let alone know what makes them work.
That most certainly does not mean that i claim to know everything about these motors, i don't and never shall do so.