Like you I use Dextron III in mine simply because it is the follow on to the original Hydromatic Type A that was specified for these transmissions back in the day.
Just now there are a huge number of different transmission fluid formulations. Seems like every manfacturer has it's own special fluid. Having said that, there are only really three types of transmission fluid and they are;
1. Friction modified and tinted red.
2. Un-modified and and tinted red.
3. JD-20 hydraulic oil. It is not tinted any color.
The original Typa A, and all of it's follow on fluids are friction modified. They have an additive package that makes them "grippier" which enables the transmission to lock up tighter. Just about every fluid that has a trade name that ends in "on" ie, Dextron, Mercon, etc, are enhanced. It is tinted red so if it leaks you will be able to know what it is. Early automatic transmissions tended to leak a bit.
Unmodified and tinted red is the old trusty Ford spec Type F.
JD-20 is a John Deere spec 20 wt hydraulic oil that is used in just about every thing from modern power shift transmissions to implement hydraulic systems. The Case backhoe on the trailer next to you at the stop light? it is full of JD-20 every where except the engine crankcase. So is every forklift, boom truck, stick crane, truck lift gate, the power steering pump on your car, and the trim tabs on my boat. It is wonderful stuff that mixes with anything.
What oil you should use is dependant on what the friction discs, and clutch plates in the transmission are made out of, and how close the clearences are. It is also dependant on how hot you are going to let the transmission get, and what kind of line pressure you are going to run.
Good ole Dextron III that works so well in our Paragons works because the clutch plates and brake band were designed for it, and because we only run 60 PSI of line pressure. The fact that normal transmission temps in our application don't get much over 160 degrees F helps a lot too.
Dextron III in a Clark-Hearth long drop 4 speed dragging a 80,000# container handler around is a disaster. Normal operating temp for it is right at 200 degrees, line pressures are at 250 psi, (the lubrication pressure, the oil that is oiling the bearings and running through the coolers is 60 to 70 psi) and in that application Dextron will glue the clutch discs together so the thing is stuck in gear in about 500 hours. (Seems like it is always third gear reverse)
If you put Type F in the boat, the shaft would likely stop creeping, but, you would be creating a host of new problems. It would not shift into reverse as quickly, and eventually it would start to slip in forward. Ford always ran a bit more line pressure than GM or Chrysler in their transmissions, and a lot more than our Paragons, so they did not need, or want, friction modifiers.
One of the hot setups back in the '60s if you were street racing a GM or Chrsyler built car with an automatic was to jack the line pressure in your transmission up to about 260 psi, then change the fluid to Type F. The tranny did not last very long, but with out the friction modifiers in the fluid you got instant, and really firm gear shifts. At $20 a gear between this light and the next one a guy could make more than enough to overhaul the transmission every month of so.
JD-20 in a Paragon would be a lot like the Type F, it would take a bit longer to fail however, JD-20 is a 20 wt. oil, and Type F is only a 10 wt. oil. The thicker oil would keep things going a bit longer.
The only thing we need to worry about is new formulations of Dextron, and what they do the the additive package. I take some comfort in the fact that here are a slug of GM and Chrysler cars from the fiftys and sixtys out there, some of which are now worth well into the top end of 6 figures. The old car lobby will insist that someone make transmission fluid for them. If it will work in a '67 GTO, it will work in our Paragons.