First of all we need to know what car you have. The older Mazda's auto transmissions required changing every 30k miles. The oils they use now are better , so probably will last longer. The best thing is to call Mazda and ask, that way you will get a straight answer. ThanksLooking over the owners manual there is no listing for frequency of transmission fluid replacement service. Can it be that it needs NO service?
Maybe we just have different opinions on what constitutes "good shape". I'm definitely not rough on the gear box. Sure, I drive the car spiritedly, but I dont slam shift or anything like that and I double clutch and revmatch when downshifting more than 1 gear. I'm used to my wife's accord where after 30k miles the ATF is still red and pretty much clear. Maybe you got a better transmission in the lotto, or maybe you would have looked at what drained out of mine and said "looks good to me!"Well, the MTX doesn't always need it. I changed mine at 50k and the oil was in good shape. A bit dark but not bad, no contamination worthy of note. I too went with the Redline and like it, but I could have easily gone to 100k on the MTX oil without a problem.
On the other hand it IS cheap and easy to do, so why wouldn't you on a reasonable interval? 50k miles ought to be fine for that interval though.
If you're having to swap it at 30k mile intervals IMHO you're being pretty rough on that gearbox and might want to rethink that.