Triumph Repair: Oils, hypoid gears, copper components

Question
I have read a bit about oils for my 68 Spitfire MKIII... 1.  Please confirm that GL5 is not suitable for the Diff due sulfur corroding copper components. Is GL6 suitable ? What modern reaily available oil is suitable - I cant find any GL4 80/90 at automotive or farm retailers.
Confirm any GL rating is OK for the gearbox (I think it has soft metal as the synchro rings)  - some say use engine oil - recommendation please.

thanks.

Answer
Hi Martyn,
I Worked at several Triumph dealerships back when Triumph was alive and they used several different brands of SAE 85 or 90 Hypoid gear oil. It is not a limited slip or anything exotic so just get an oil that is meant for Hypoid gears.

As far as the trans. I have seen just a plain 75 up in gear oil used in Triumph gearboxes. If you are in the far north you may want to use engine oil because the syncros tend to not grip with heavy oil. Even in Florida we at times found that engine oil worked better. We also had MG and we always used engine oil in their transmissions all through the BMC years and along came BLM and they told us to switch to gear oil and we were covered up with shifting complaints so we switched back to engine oil long before it became clear BLM didn't have a clue about much of anything. While it was true that we seen less complaints in Triumph using gear oil in the trans, we still had Auto Cross customers who used engine oil in them.
Howard