Re Hypoid oil,     I was under the understanding that the main difference
between GL-4 and GL-5 oils, apart from viscosity, are additives which make
the GL-5 an EP or Hypoid oil.   These I believe to be the very additives
which can damage bronze parts and are therefore not recommended for manual
transmissions.
It should be noted that on automatic transmissions with a separate oil supply
for the final drive, GL-5 is recommended.
This information is fairly dated and it may be that it is incorrect for some
modern oils.
Personally I think it unwise to depart from the VW recommendations.
The recommendations of BMW and other German manufacturers concurr.
Steve Cotsford