Personally, I would be more concerned about the pressure rating (GL rating) rather than the viscosity.
GL rating is based on how much pressure is exerted between the gear teeth as the gearbox is under load.
I would most definitely avoid GL-5 for the gearbox (also known as hypoid) which is extremly high pressure for differentials, but I would put that in the back axle of an Amazon.
I've been using this product (GL-4) from Millers for all my old cars having read a few nasties of using extreme high pressure gear oils in VW aircooled gearboxes:
http://www.opieoils.co.uk/p-68916-mi...ep-80w-90.aspx
I have to say that since changing from whatever was in my old Beetle's box to this has resulted in really nice gear changes!
With reference to viscosity, 80 was recommended, but I don't know whether multi-grade gear oils were available back then. I like the idea of 80/90 for good viscosity in a range of temperatures.