Why does my car indicate 5W-30 is good year round oil but many viscosity charts say it's only best under 60F?

[SC Maintenance]

Probably because the manual was written at a time when 5W-30 was of a very poor quality and not very shear stable and would break down and suffer from base oil evaporation at elevated temperatures, in a water-cooled car engine modern 5W30 should be virtually no different than 10W30

So is there any validity in this anymore at all. Or more specifically, if you live somewhere that never gets much under freezing, should you just run a 10w-30 as it would be less susceptible to shear?