The issue here is mainly one of excessive viscosity at low temps. Most of the 75w-90, GL-5 rated gear lubes are formulated fairly thick in order to provide good protection in differential applications. This impacts their ability to provide good shifting in a transmission application at very low temps.
The API 90wt gear lube spec allows a lot of leeway in terms of viscosity. If you compare the GL-4 rated, Amsoil transmission/tranaxle oil to their 75w-90 Severe Gear for example, you'll see the Severe Gear is much thicker at all temps, but especially so at lower temps where shift quality is an issue.
Just to clarify:
NV4500 transmissions use the 75w-90, GL-4 rated Amsoil transmission fluid
NV5600 transmissions use the 5w-30, Amsoil MTF fluid, which does not contain extreme pressure additives, ie sulfur/phosphorus compounds.
TD