As a winter skier and winter tourist.....
Jackson Hole Wyoming, VW 412 w Mobil 1 5W20, mid 70's
-43, started eventually. Only car in lot to start.
Colorado, Several events at -40, always Mobil 1. At high altitude little power is developed , once I had to heat the block with a coleman single burner to make it happen.
Ely Minnesota, a few years ago, -47f, 3.1 litre Lumina apv, Mobil 1 0W20. Absolutely the only vehicle in the area that started that was not plugged in. Engine howled, growled screeched and screamed but eventually all was well.
Failures: Most of my cars stop starting when the cold CCS reaches about 30,000, at this point they pop and fire but no longer make enough power to overcome viscous drag. With 50 years of data, here is what I have:
20W50 Cars stop starting at about - 8f
10W30: Cars stop starting at about -25f
5W30: cars start down to about -34
5W30 Synthetic... fail at about -40
0W20 Synthetic: still starting at - 47F
Reading product data sheets - these numbers seem consistent with the anticipated viscous drag of the various oils.
This also nicely matches one of the first SAE journal
research articles that I ever read in the U of Iowa engineering Library - conducted by General Motors in about 1962.
I have seem some "polar start" oils listed for near arctic areas but have never had opportunity to try them - or no need to I hope!!