Price
Redline MT-90 is a great gearbox lubricant for 'boxes that have to work hard. It's not the best choice for street cars because it needs to be up to full temperature to work properly, but if you're doing a lot of track work or need a little more protection for your gearbox it's the one to choose.
Recommended for street and track driven NDs. For street NA/NB/NC cars we recommend MTL.
NB 6-speeds require two quarts, all other Miata transmissions require three quarts (the third quart is used in the shift turret).