So yes if you're running continuously for months on end in a cold climate you can certainly benefit from running a 0W-20. Of course having oil gauges is best to monitor things if you're going to deviate markedly from the lightest oil specified by the manufacturer.
But my point is that this the requires everyone who is operating the car to understand the limitations of what's in the sump...and not take it on the emergency funeral run without doing an oil change.
My Caprice has a big winged alloy sump, and still hits 105C oil temperatures (type K thermocouple dropped down the dipstick) when it's freezing, in as little as 20km on the highway. (before you start, that's a 3.2cst HTHS 5W30)
As an example of my argument, I'll use something that you are familiar with, the owner's manual of the Nissan 510.
Clearly, 5W20 was acceptable (not a recommendation) at continuous operation at ridiculously low maximum temperatures. It's similar to your assertion in this thread.
SAE40 was acceptable (not a recommendation) at continuous operation at ridiculously high temperatures...reverse argument to yours, but still applicable.
But the Goldilocks recommendation (and I say that because it's at the top of the list and covers all of the temperature ranges) was 10W30, which rather than being you "belt and braces", was just more suitable for "normal" people, doing normal stuff with their cars, all year round.
You don't know, I don't know, and CATERHAM doesn't know where on the curve the BMW engineers set their operational parameters...they clearly agree that the moving surfaces should not touch, but where to the right of not touching they are, we just don't know.
They have designed in a safety factor "belt and braces" as some call it...we don't know how big that happens to be.
To unilaterally declare that viscosity can be lowered over what the OEM says is arbitrary, and not science based...lack of blow-ups is not science.
