I am changing the oil in my 1951 1/2 ton, 216 ci, for the first time and would like to know what weight oil is recommended.

My manual calls for 20W oil for temps down to 10 degrees. This information is from a 60 year old manual, so I would like to know what weight oil is the best to use by today's standards. The truck does not have an oil filter and is not road ready at this time. I will not be driving the truck during the winter but will start it weekly up to keep it loose.