Any suggestions which would be the best Hypoid gearbox/ diff oil to use in a 1970/ 71 Beetle ????

80 or 90 grade its all the same and smells of fish
make sure you rotate the wheels when filling to get all the new oil into the diff and let it drain for a hour or so very heavy oil so it moves slow

and clean the magnetic drain plugs

Depends on the average ambient air temp as to the weight of oil used but my Gregorys manual says 80W90 is a good inbetweener.