Look at the bottom of the transmission... if it has a rectangular removable oil pan, it is NOT an M5R1. If there is no pan at all, it is an M5R1.
The only Ranger in 1989 that had an M5R1 from the factory also had a 4 cylinder engine, 2.9 powered trucks would have had Mitsubishi transmissions.
If you do indeed have an M5R1, the correct fluid is Dex/Mercon II/III ATF. The Mitsubishi transmissions will use Pennzoil or Valvoline MTF/Synchromesh type fluid. O'Reilly has it, about $10/quart. There are other fluids that will work in those but that is the best one I have found.