Triumph Repair: Lubricants, synthetic gear oil, haynes manual

Question
Hi Howard,
Pair of questions for you.  I've purchased synthetic gear oil, 70w90 in both GL-5 and GL-4.  I read that one cannot use the GL-5 in the rear differential as it will corrode a copper bushing.  Can you confirm that the GL-4 is safe to use in the rear diff?  The Haynes manual indicates that on later models of Spitfire there is no differential drain plug, and it seems to be the case on my car.  Is there a way to drain the diff short of removing it and tipping it over?  Or would you say that it's unnecessary and the 36 year old oil is probably still fine.  And finally, can you please confirm that the GL-5 oil is an acceptable product to use in the gearbox?  
Thanks as always.  Looking forward to your reply.
Best Regards,
Eric

Answer
Hi Eric,
There are several Pros and Cons on using GL-5 when GL-4 is recommended. Some claim it is the heating of the sulfur content in GL-5 that causes the copper problem but from what I can find out, this is not the case in a synthetic oil. Some claim better lube with GL-5. From all I read it is your decision. If it is true that GL-5 in a synthetic does not contain sulfur I would not worry about using it in a differential and use GL-4 in the gearbox.

To add more stuff in the game from my experience in MG, Tri, Jaguar and other British cars and American cars, Many operate much better as far as the gearbox (transmission) goes by using engine oil.

Example, We in the dealership were using engine oil in the gearboxes and when BLM took over they told use to quit doing that and only use gear oil. Within weeks of changing back to gear oil we had dozens of cars lined up every morning complaining of gear clash. Especially starting off with cold oil in the morning. (the syncros wouldn't work) So we had to ignore BLM and go back to engine oil in the transmissions. We always used the GL-4 in the differential as recommended. But there were no synthetic oils back then.

Even if the books say GL-4 in the gearbox and diff. I would not worry about using a synthetic GL-5 in the differential and GL-4 or engine oil in the gearbox. If you presently are using GL-4 in the gearbox without any morning gear clash, I wouldn't bother changing it.

I have no idea what BLM had in mind when they started not putting drain plugs in gearboxes, differentials, radiators and gas tanks. If they had anything in their minds at all. While it is true that you can't ware out oil, you can contaminate it in a gearbox and differential.
That's my two cents on the matter,
Howard