According to the original 1948 owner's manual, my truck was delivered new with 10W oil to be used for the first 500 miles. After that, these were the recommendations depending on the lowest temperature the truck might see.

Down to 32 deg: 20W or 20
32 to 10 deg: 20W
10 to -10 deg: 10W
Below -10 deg: 10W plus 10% kerosene (must not have had 5W in those days)

I use 10W-30 Mobil 1 year round. That was also my break-in oil for the 261.

The manual also has a nice explanation of the difference between the 20W and 20 designations (viscosity determined at different temps but that's probably on the internet).

Last edited by Wally / Montana; 04/30/2020 11:43 PM. Reason: Addition about break-in
1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.