you could try this,
http://chevy.oldcarmanualproject.com...esto/index.htm
maybe a different year but same style truck would give the lube type if iy doesn't show for your year. multipurpose gear lube likely, 80/90 weight gear oil. the problem is it will leak out if the bushings and seals are not in good shape. some use grease instead but that could get stiff in cold weather.