After 50+ years of messing around with cars, I have come to the conclusion that motor oil is motor oil...
The cheapest oil you can buy today is better than anything available in the 1950s. As for the recommended viscosity or weight, this is what the 1955 Owners Manual reads:
http://chevy.oldcarmanualproject.com/chevyowner/55om17.htm Given that I would say any 5W30 or 10W30 would be plenty adequate for year round service in Missouri, or anywhere in the lower 48.
