I found that Toyota T-IV transmission fluid is also used in Mazda's 6 speed Auto. transmission. According to a Mazda bulletin the following fluids are compatible with the Toyota T-IV transmission fluid. ATF type JWS 3309 produced by Mobil and Exxon.
Mazda also adds that this fluid (JWS 3309) is available thru GM part#GM9986195, MOBIL JWS 3309, FORD XT-8QAW, and Toyota T-IV.
Mazda North America adds the type of transmission used in their vehicles is one that is manufactured by Aisin Warner. I think the U140E/F transmission used in the RX300 is a Aisin Warner transmission.
So I'm under the impression that we can use something that Toyota's type T-IV.