I agree with Dennis...GL5 does not seem to degrade the syncro's enough
to be horribly concerned. While running a repair shop for many years, We
used GL5 synthetic and never had a premature wear  issue that we were
aware of in a tranny. The hard parts breakage of the 3/4 slider hub and
pinion/mainshaft bearing failure is much more common that a syncro
problem. I personally like and use Redline due to its good cold shifting
properties. Sometimes a tranny will "need" a thinner oil such as Redline
to shift easily. In that case...use it. If it seems to work ok with the
75/90 synthetic, use it. I have seen very little difference in breakage
or wear between the two types of oil.
The key in my opinion is to change the stuff once in a while instead of
leaving it in there forever...The big thing I see is that people who are
concerned enough to use a oil like Amsoil or Redline, is that they WILL
change the oil often and the insides of the gearboxes show that. I
recommend every 30K mi. even for Redline. The long chain molecules in all
the varieties of oils do wear out (as I have been told) and fresh oil is
cheap insurance.
My unscientific .02 cents worth
Daryl of AA Transaxle
Duvall, WA. (Seattle area)
(425) 788-4070
1-877-377-0773 (toll free)
aatransaxle.com (web site)