Originally posted by badkid
MT-90 is a 75W90 gear oil and is of the proper and recommended viscosity. It's GL-4 and thus is not as slick as the GL-5 75W90 oil.
GL-4/GL-5 has more to do with the ability of the lubricant to handle high pressures and shear loads without falling apart. The GL-5 has more extreme pressure additives.