Worth mentioning - Rotella T6 is now available in 0W-40.
API SN service classification. It's slightly thinner than the 5W-40 at all temps, but still thicker at operating temp than Mobil 1 5W-30.
I haven't really noticed a difference in cold starts. Engine might be less groany at less than -20. Could also be my imagination. Same price as 5W-40, and strikes me as a "may as well" for winter use.