MT-90 is much better than MTL for my MSM 6 speed.MT-90 is the Mazda (and Red Line) recommended viscosity, and I agree.
The previous owner went with MTL (28000 miles) in the gearbox and I've always found 1st to 2nd notchy until the car was up to temperature.
I just installed MT-90 (61000 miles) and all is well.
This is the oil that Red Line recommends for a Miata 5 speed box.The oil that Red Line recommends, on their website, for our gear boxes is MT-90 75W90 GL-4 gear oil. Our gear boxes were designed for convention (non-synthetic oils) and the clearances are relatively large, larger than for a modern box. A correspondingly heavier oil works well in our boxes, especially for severe applications and boosted cars.