Not that this is of any real help to the original poster, but this is what the 1953 Chevy Truck Owner's Manual was recommending. I'm not sure that I would want to use a 20W or even a 20 oil in a truck in the summer.

When I was working in a Texaco station in the late fifties, we stocked a 10W, a 20W-20, a 30, and 10W-30. We did not stock a 20W or 20. There was still a lot of mistrust of "multi-viscosity" oils such as the 10W-30, but not of 20W-20. As I recall, the 10W and 30 oils were labeled "ML-MM" and the multi-viscosity oils were "MS-DG", but my memory may be playing tricks on me.

Lots of interesting information has been posted this time.

Hoyt