ATF is the lifeblood of your transmission. If the maintenance handbook requires you to put original Mitsubishi ATF, then by all means follow it.

Never scrimp on which ATF is the cheapest or more recommended. Too late to realize that if your tranny is already slipping and sliding in gear, or it wont shift at all.

Lastly, have a tranny flush, not just an oil change. If your gearbox has an ATF filter, replace that too. Clean, if its the screen/mesh type.