Redline MTL is a 75w80 synthetic has been the go-to fluid for Miata owners since the beginning. Smooth, accurate shifting due to the special additives for the synchros. It's what we use in most of our own cars.
Pure track cars should use MT-90 - it's not as happy when it's cold, but it will deal better with the high temperatures of the track. We also recommend the MT-90 for ND Miatas, as it is very close to the stock formulation.
For maximum gearbox protection on hard-driven track cars, mix in a quart of Shockproof gear oil.
NB 6-speeds require two quarts, all other Miata transmissions require three quarts.