There’s a guy in my class who has over 200,000 miles on his vehicle and he has never changed the trans fluid and it’s still operating just fine.
I drop the pan and change the filter on all my vehicles and replace whatever fluid is lost. I have never had a transmission flush done.
Service information on most vehicles will state that transmission fluid should be flushed at regular intervals.
Most shops are too chicken shit to perform a transmission flush because they feel there is too much liability involved.
Clutch material dissolved in the transmission fluid add friction to the clutches and help the clutch packs grab during clutch application. If you had a total transmission flush, there would be no clutch material in the fluid at all and the clutches would slip more.
So.. after all of this, the moral of the story is “who knows.” Nobody has a clear answer on this. There are too many variables involved in order to determine if it is ok for YOU to flush YOUR fluid on YOUR transmission. Therefore, we err on the side of caution and either advise not to change the fluid if it “hasn’t been changed in awhile,” or just drop the pan, change the filter, and replace what fluid was lost from dropping the pan.