[clip clip, lots of folks wrote]

Very confusing issue and discussion.  Just a few brief points:
1)  The German mechanic that installed a rebuilt transaxle for me 9
years ago said "use GL-4 oil only, NOT GL-4/GL-5, GL-4 ONLY. Wrote it
that way on the invoice.
2)  GoWesty is not everyone's reference of choice, but they still
recommend and sell MT-90.
3)  The Redline website has an application guide: plug in your vehicle
(for me an 87 2.1 hardhat) and the recommended transaxle oil is MT-90.
4)  Following a discussion like this about 6 months ago, I called
Redline and the person I talked to knew nothing about a formulation
change and could not imagine why one of their reps would say MT-90 is
not recommended for our vehicles.  When I asked about the "not for
hypoid differentials" statement on the site, he said Hypoid
Differentials are not the same as our Hypoid Transaxles and went into
some stuff about shaft angles and pressures and whatever that was way
over my head.
5)  A  clip from a dandy 2-page technical article on Redline's website:
      "GL-5 gears oils which are required in hypoid differentials are not
used in most synchromesh transmissions because the chemicals used
to provide the extreme pressure protection can be corrosive to
synchronizers, which are commonly made of brass or bronze. Typically,
the use of a GL-5 lubricant in a synchromesh transmission will shorten
the synchronizer life by one half."

So ... I ain't an expert, but I did enough homework to be comfortable
continuing to use MT-90 and to throw out a caution about any GL-5.

Your wives' tales may vary ...