id recommend Silkolene Pro R 10W-50
you still get good cold performance and good high temp aswell
however ive found Elf Competition ST to still give excellent cold weather performance, something ive never experienced with a 15W-50 before, always found Mobil 1 Motorsport to take a while to get flowin in severe cold, not sure what it will be like on the GTT lump
the lower W number the thinner in the cold, which to an extent is a good thing however you might not want to go too low with a higher mileage engine
depends how you drive your car really, if it gets thrashed a lot get somethin with a 50 as the last number