Higher altitude flight, sure, very low temps. -70F not uncommon. -40F common all the time.
But on the ground, for me, -65F.
And in 1977, I had a diesel Rabbit, which I managed to get going in -38F conditions. It was in upstate NY. The battery went inside over night.
I tried to use a propane torch to preheat the oil, but propane doesn't vaporize well at those temps. So a small camp stove was put under the oil pan. 30 minutes of heating with that, and a "double glow" with the glow plugs and the engine started up.
In those days, the best defense against fuel waxing was to run #1 (kerosine), which on that day I had a 50/50 mix of in the fuel tank. We tended to leave the fuel tanks about 1/2 full, so that we could pour warm fuel, kept inside overnight, to de-cloud the existing fuel if needed.
Starting in -20F was not too much of a problem, but -38F was a challenge. Also that was back when the lightest oil used was commonly a 10W-xx.
I have friends in Canada who experience -30F and -40F temps in the winter and own VW TDI diesels, and they have comparatively few challenges starting them. The engines are in general easier to start, and reformulation of diesel fuel has reduced the fuel waxing / clouding problems.