[WopJob]

My 2 cents: I just put Redline MTL90 (75W-90) in my spider trans to help with extreme cold weather drivability. It helped. I can change gears in 35F degree weather without waiting for 45 minutes before driving off. I had Spirax 80W-90 before. Both are recommended for spider trans. There was nothing wrong with the Spirax in the summer.
I know that this does not answer your question about the GL4 vs GL5 dilemma, and definetly does not put the issue to rest. I'm not sure that we can find the differences between the two (who wants to try and post the unobjective results?). there will always be some question of objectivity in the research. That's why the issue of OIL, coolant, and brake fluid will not go away.

ps. Nissan was mentioned before, what year and model are we talking about?

 

[WopJob]

...what material are the synchros in the Spider gearboxes made of?

 

[WopJob]

They're molybdenum. Everything in the modern Alfa tranny is GL-5 compatible - no brass.

...So, since the additives in GL5, whatever they are, attack (coat) yellow metals such as brass we should not use it. But since there is no brass or any other yellow metal in our gearboxes then GL-5 os OK to use, just as the manual say.
Additionally, since Gl-4 works with Molly then it can be used too.

case closed?