Transmission Fluid Replacement - DIY or Dealer?

There are strong options that parrot the Toyota dealer line of requiring Toyota OEM fluids only, yet no one can state why (beyond the Toyota corporate line of 'specially formulated' which every bottle of fluid from every manufacturer states.)

Toyota is not in the fluid's business. They are not creating (nor formulating for that matter) their own trans fluids, coolants, oils etc. They are rebranding it, from another vendor. The trans fluid could easily be a Valvoline or any other product w/Toyota sticker.

For all the faithful cries insisting on Toyota fluids, no one has stated why beyond quotes from Toyota or personal beliefs.
For all the calculations that have been projected between the extra cost of Toyota OEM fluids to the cost of damaged Prius engine equipment, no one has a story of anyone who ruined their vehicle by using a substitute fluid ~ no stories of shortened life span, performance degradation. A few folks have, though, trepidatiously jumped in to state they have been using substitutes for years w/no issue.

Example: I use a full-synthetic oil and replace it twice a year, for over 8 years now (no issues.) Synthetic oil is synthetic oil, regardless if Toyota pushes for it's own branded oil, which likely also has its share voodoo threats and risk pushing the sale of OEM purchases.

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Status: I did use OEM and found a dealer who's price was quite close to the 3rd party. I am here, all the same, to flush out WHY. Why OEM WS only other than Toyota says so? Toyota says use only Genuine Toyota parts too, however 3rd party parts specific to the Prius work great.

 

Hmm... so this interesting. Watched a few videos which all used the same method....get the car level, fill until it starts leaking.
I filled 3 bottles.... then every few pumps w/the 4th bottle I would wait a few seconds/minutes expecting dripping to start. Car was level.
Eventually, the entire 4th bottle made its way to the trans fluid container...it even seemed as though there was room to spare. I removed the tube, left the plug off while started cleaning up a bit and then, all of a sudden out of no where, the fluid started leaking. Leaking as expected, however minutes after the tube was removed. I let it drip for a while, and while I was expecting the drip to slow down/stop once leveled out, it was is if that wasn't slowing so I eventually plugged it back. I expect about 10% of the bottle dripped out.

Test drives seem fine... how would I know if it has too much (or too little)?

 

So what happened? Break down? Did the dealer agree to change it out?