This has been an interesting thread.
I have never tried ATF.
I have disassembled hundreds if not thousands of dirt bikes.
I have seen ATF to the newest synthetics and everything in between.
The bikes that come in that are clean and the air filters clean seem to be clean on the inside as well, regardless what is used as a lubricant.
The bikes that come in with caked on, baked on who knows what, with a air filter that looks like it was used as a chew toy for a pack of Rottweilers in a mud pit... typically they are very nasty on the inside.
True, the ATF bikes are very clean on the inside, usually a little cleaner than the more conventional lubed bikes.
Below is a pic of a YZ125 we were working on today that ATF was used in.
We haven't done any cleaning.
What you see is what it looks like immediately after the ATF was drained and the engine was disassembled.
The bike was true to form for a well maintained bike.
Yet, as far as gear wear.... I have seen no real difference between ATF and any other lube.
What seems to be the biggest factor to gear wear is the gear lube not being changed often enough.
The second thing we see is poor shifting habits.
So, what ever you use, change it often and pull your clutch lever all the way in, shift the bike in a positive manor, and then let your clutch out.
Your bike will live long and prosper