Is it true that in Mitsubishi's requiring Mitsubishi diamond sp3 if you dont use factory fluid is causes harm?

People are saying if you use fluids like royal purple atf amsoil atf it will hurt the transmission and this is what the hyundai and mitsubishi clubs and mechanics are told.|||As long as you use the same weight and specification lubricants the factory recommends, you can buy any brand you want.|||Royal purple isn't really that great, alot of the guys at my car's forum prefer mobil 1 or amsoil as far as engine oil. For ATF it's usually better to go with what is recommended as far as I know.