Wrong kind of gear oil = ?

[probedude]

Steven,
I've read that GL-5 has additives that are corrosive to bronze metals, of
which synchros are typically made from.  If Motuls says their fluid can be
used in GL4 and GL5 applications, then I would guess that they do not have
these corrosive additives so it will work in GL4 applications  yet still
meet GL5 applications.

However that said, I don't think it is correct to say that GL5 is an
improvement over GL4.  They're just two different lubricant names used for
different applications.

Someone correct me if I'm wrong.