I called Redline a few months ago - as summarized in the Tranny oil thread.
viewtopic.php?f=43&t=19619&start=45
In an application that allows either a GL-4 or a GL-5, 75W90 viscosity we recommend the MT-90. If your factory owners manual does specifically call for a GL-5, I would recommend the 75W90NS.
Some GL-5 gear lubes can be corrosive to brass, bronze or copper also many are too slippery for the synchros. Our GL-5 gear oils don't contain sufficient EP additives to be corrosive, the "NS" fluids are designed to provide good shiftability with the proper coefficient of friction for the synchros.
Regards,
Red Line Oil
It being thicker is also what Redline claims gives better shifting.
I don't think that is what they claim. MT-90 Shifts better because it does not have the same Shock protection additives as the 75W90
Also, according to the charts on their site - both the MT-90 and the 75W90NS have the same viscosities at 100*C and the 75W90NS is actually a bit Thicker than the MT-90 at 40*C
From what I understand, the "thickness" of the oil is not the defining factor of gear protection - its the quantity of shock protection additive in the oil.
Redline 75W90 Gear oil has the most
Redline 75W90NS is a comprimise - designed for use in gear boxes like ours.
Redline 75W90 MT-90 is a GL-4 which does not have the same shock protection additives. This means it is great for shifting and syncros, but not necessarily good for diff protection.
If someone has some sort of evidence or personal experience showing that GL4 is perfectly safe in our transmission, I would try it immediately because I would expect smoother shifts from the GL4 vs GL5 which is recommended by toyota - and redline for that matter when i last e-mailed them.