Get this right: the Miata manual gearbox wants GL-4 gear oil, and the differential wants GL-5. Putting GL-5 in the gearbox is the classic mistake that ruins shift feel and, over time, the synchros.
Why it matters: - The gearbox has brass/bronze synchronizers. GL-5 oils carry high levels of extreme-pressure (sulfur/phosphorus) additive meant for hypoid differential gears. That additive can attack yellow metals and, just as important, is too slippery for the synchros to do their friction job - so the box crunches, especially into 2nd when cold. Use GL-4. - The differential is a hypoid ring-and-pinion. It needs the EP protection of a GL-5 75W-90. GL-4 is not enough for the diff.
So: GL-5 in the diff, GL-4 in the gearbox. Never the reverse.
Gearbox oil choices (all GL-4): - Redline MTL (75W-80) - the enthusiast default; superb cold shifting, best in cool and moderate climates. - Redline MT-90 (75W-90) - a touch thicker; preferred in hot climates and for hard or tracked use. - Genuine Mazda MTL - the factory-blessed fluid, a safe pick. - ATF (Dexron) - some run ATF for the smoothest cold shifts, and Mazda specified an ATF-type fill in places. It shifts beautifully but shears down faster, so change it more often. - Avoid "GL-4/5" compromise oils - they do not give the synchro feel Mazda designed in.
Differential oil: - 75W-90 GL-5. A Torsen LSD needs no friction modifier; a clutch-type or viscous LSD may want the LSD additive. Fill with GL-5 and reserve the friction-modifier fuss for clutch-type units.
Capacities are modest - well under two quarts each. When in doubt: GL-4 box, GL-5 diff.