And if I take a picture of the oil chart in the manual for my 71 C10 it shows everything from 10w to 50w depending on duty severity and time of season.
They put a dozen different conditions on six grades of single weight oil in there to choose from!
And yes, I do have mine, though sitting in the glovebox for 53 years has made it insanely delicate.
Again, they're way smarter than we are and they did math we can't comprehend. If they say it needs 0w20 it needs 0w20.
Hell the only reasons I'm not running straight 30w in my C10 is because 10w30 is much more readily available and is the recommended general usage oil for 250 I6s produced after multiweight oils became commonplace. Rather convenient that it acts like a 10w when cold, though.