For the differential you should use a GL-5 oil. It’s an EP (Extreme Pressure) lubricant that protects the hypoid gears. I use Lucas 80/90. And for the transmission you can use 90 weight mineral oil or a GL-4 oil. Do not use a GL-5 oil in the transmission, it has a high concentration of sulfur/phosphorus additives that can corrode yellow metals like bronze, copper and brass synchronizers.
Below attachment is from the 1948 - 1951 Chevrolet Truck Shop Manual.
