The 40 is the warm rating, a 0W40, 5W40, 10W40, 15W40 etc all are in the same viscosity band at 100C and provide the same protection in the hot.
There is a further rating called HTHS (High Temperature High Stress) which measures oil viscosity at 150C. But if our engines get oil that hot...
Essentially as you go lower in the W ratings they have to be more synthetic. By the time you get to 5W40 they're pretty much all fully synthetic. At 10W40 there are still some mineral oils.
But it's mostly marketing BS.
The W rating on the oil tells you how it flows in the cold. A synthetic 15W40 is no better than a mineral 15W40. The average 15W40 won't even pour at -35C.
SAE ratings:
15W oils are good for -15C starting.
10W oils are good for -20C.
5W oils are good for -25C
0W oils are good for -30C...
I put a similar thing to this on my diesel ute last winter. Has an inbuilt thermostat so only heats to 60C.
Fantastic addition. I have it running on a timer for 1.5 hours before it's needed. The windscreen is already thawing, the engine starts without trigging the glow-plugs and the heater...