Use DOT 3, DOT 4 or DOT 5.1 fluid. The system holds about 250ml, but for a complete fluid change, reckon on 500ml.
There are 2 types of DOT 5 fluid (Ethlyene glycol based & Silicon based), neither is suitable for MX5 braking systems.
Mazda only recommended the use of DOT 3 brake fluid until the issue of a TSB in October 1997 stating that the material used for the flexible hoses for the brake system had been changed at VIN 100020 (November 1989 production) to allow use of DOT 4 brake fluid. DOT 3 fluid must be used for vehicles before that VIN. Anyone got an MX-5 with a VIN before 100020?