If you wanna know the best advice. I got from a nascar racer that worked on transmission's/engines/etc. his whole life.
If the fluid doesn't smell burnt and the color is good. Don't ever change your fluid. Your just askin for problems
Sorry, but I disagree.
I believe the OP is talking about manual trans, which in this case I feel it's most important to drop the fluid to clean the magnet on the drain plug. Keeping from metal circulating throughout the trans if the magnet is full, since the occasional grind will happen.
As far as an automatic goes, if the auto has been running for 70,000 miles without a service and is suggested by the manufacture to be serviced at regular intervals...I'd agree on not changing it. For one reason, the fluid starts to carry clutch material and can actually cause excessive clutch wear while aiding the clutch holding as well. There is a point where that material in the fluid actually becomes vital because the clutches are so far worn that putting clean fluid in it will promote slipping.
Regularly servicing an automatic allows the filter to remain clean, promoting good flow. The pan magnet to be cleaned, to prevent excess material from traveling through the trans and there is without a doubt no advantage in any hydraulic system to run "dirty" fluid. Period. Clean fluid will lubricate better, clean better and transfer heat better (which is the #1 killer of autos). Now there are other transmissions (mostly European based or CVT) that have "lifetime fluid" in them. These don't share a whole lot with the 4-speed in a Wrangler and the fluid is about $30/quart that has very different properties than a basic transmission fluid....which is an entire other discussion.
I'm not sure if I understood your credible source was a "racer" or a NASCAR trans guy...but there isn't a whole lot in common between a stock car that gets rebuilt after every race and a vehicle that gets driven everyday. The "best advice" certainly depends on who you talk to. An engineer's opinion, backyard mechanic and a racer's opinion will vary drastically. I'm just a Master ASE and Master BMW dealership tech gone back to school for an engineering degree with my opinion...so whatever that is worth is up to you.
- I do not suggest NOT regularly servicing an automatic that is designed to be serviced, period.
Just my 2cents.