Recently I had to replace my pilot bushing due to dry condition. When removing the Trans I drained the oil and replaced the original MT-90 with Royal Purple 75-90 gear oil and a few drives later it started getting a slight grinding going in 4th gear both up and down shift. I decided to change it back to MT-90, and lord and behold the shifting problem is gone. I Think MT-90 is the right oil for the Z.