The factory actually specifies 90w mineral oil (page 0-5 of the 1948-1952 Chevrolet Truck Shop Manual).

Go to Tractor Supply or a similar store and purchase 90weight mineral oil. It's sold as Ford Tractor Transmission Fluid.

https://www.tractorsupply.com/tsc/product/traveller-ford-tractor-hydraulic-oil-2-gal-t806600-1636284


1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)