GL-5 is a rating meaning the oil is suitable for heavy duty use in differential assemblies with hypoid gears. That is almost anything from 1940 on (and in some cases much earlier). It should be the right thing to use in Chevrolets of the 40s and 50s.
It may not be safe for brass or bronze parts, so if your differential has any brass or bronze in it you need to check compatibility with "yellow metals" for the oil you plan to use. There should not be any yellow metal in the differential of a 40s-50s Chevrolet car or pickup truck, as far as I know.
I've not heard of seal problems, but old seals could be on their way out anyway.
Last edited by bloo; 06/24/20 08:33 PM.
