For an NB, a conventional ethylene-glycol coolant is correct. The NA and NB (1990 to 2005) predate the long-life formulas, so a quality green coolant mixed 50/50 with distilled water is the safe, standard choice. Distilled only: tap water leaves deposits.
How much: the capacity is modest, roughly a gallon and a half of total coolant on these four-cylinder cars. Buy a gallon of concentrate and mix it.
Two rules I hold to:
- Do not mix chemistries. The NC and ND use Mazda's long-life FL22, which must not be blended with old green coolant. When in doubt, flush fully and refill with one type.
- Bleeding is mandatory. The Miata traps air high in the engine. Fill slowly, run it with the heater on full, and burp until the level stops dropping. If you skip this it will read as overheating.
Interval: refresh conventional coolant every couple of years.
Colour is a guide, not a spec. Match the chemistry your generation calls for and keep it clean.