[AT] Tranny Lube

[The Allen Family]

I have a question about transmission lube for my JD A and B.  Both are
overdue for a change, but I want to choose wisely because I don't want to
waste any.

What is the best lube to use?  I have read conflicting information.  Some
sources say to use a GL-5, but others say that some of the additives in it
are harsh on brass and copper.  I'm not sure that these tractors have any
brass or copper in their transmissions, but I worry about taking the chance.
Does anyone know whether GL-3 has the same additives?

Another source says that SAE 50 oil is the same as SAE 90 Gear Lube, which
doesn't sound logical, but the source appeared to know his stuff.  Can
anyone confirm?  Does anyone know of a source for SAE 50 oil?

The quantities involved will cost well over $100 for the two tractors, so
I'd like to make sure I use the best choice.

TIA for any suggestions and relevant info.

The "original" Steve Allen