Acknowledging it is allowed to slip a grade in service, no lower than the CCS temp, which is -25C.
Which would be -13F and still pretty darn cold.
Realistically, most of us in the lower forty eight will never see temperatures cold enough that a 10W grade oil would be a problem.
Even lower overnight temperatures won't usually matter since they typically aren't sustained long enough to chill the oil in the sump to the ambient temperature of an only briefly reached low.
There is also always the option of a sump heating pad, which makes the W grade of the oil irrelevant.
One of these would also be nice on one of our below zero mornings, since warmed oil makes for a warm engine and faster heat to demist the windows and warm the driver without a lot of idling.
You don't see these much here on gassers, but I did see the tell-tale AC hanging out on local gassers on a drive through South Dakota and Montana.