First of all I want to state that I'm no engineer and the observations here are from personal experience and 40+ years as a auto and bike repair shop owner/mechanic. In the last 30 years I've built well over 1000 automatics for every application from stock to all out race applications in 1000+hp race cars and monster trucks.
For those of you (including myself) that use ATF in the primary here's some facts that I have verified over the years.
There are approximately 25 types of ATF but I'll cover the 5 most common as the other twenty are either obsolete or very vehicle specific.
Type F or (Ford) fluid was developed for and used in Ford automatic transmissions back when the clutch discs were made of sintered bronze or asbestos and there were no bearings used. All parts were supported by bronze bushings and thrust washers. It is essentially 20wt hydraulic oil (pure mineral oil) with a red dye and that's it. It is commonly sold as Racing ATF under brands such as B&M Trick Shift because it provides the quickest lockup of the clutch packs (Hardest shift). It has no place in 99.9 percent of modern applications unless you're just looking for neck snapping shifts (or in the case of motorcycles used for drag racing where quick and harsh clutch engagement is the primary goal and you're gonna change it very often.
Dexron II and III are the most commonly used fluids with the only difference being additives in Dexron III to make it more compatible with the electronics used in modern transmissions.
They both have an additive package that includes friction modifiers for smoother cluch engagement, oxidation control, viscosity stabilizers, corrosion inhibitors and proper lubrication of bearings and bushings of all types. Since the advent of gerotor pumps and better friction materials such as Kevlar and Aramid, Dexron II or III has even replaced the use of Type F for racing use by most transmission builders.
Dexron III is commonly used in many automotive manual tranmissions and is probably used in more transfer cases of 4WD vehicles than any other fluid.
ATF+3 uses higher quality base oils with the highest content of friction modifiers to prevent torque converter shudder. It lubricates a bit better than Dexron III but can also cause slow clutch engagment (slipping). If you ride easy and your primary goal is smooth clutch engagment then this is the fluid to use in your primary if you're gonna run ATF.
Synthetic ATF comes in many formulations but the most common is Dexron III. It has the same properties of clutch engagement as Dexron III with some advantages.
The main advantage of synthetic fluid is it's ability to resist thermal breakdown. If you ride in extreme heat or just want to extend your primary fluid change intervals then this is probably your fluid.
You may want to think about this too. Although the primary doesn't generate much heat in and of itself that's not the only concern when choosing a fluid.
The primary on your motorcycle is not a seperate entity. It is attached to the engine and it acts as a giant heatsink for that engine. The temps seen on a long hard run are probably within 20-40 degress of the engine oil temp.
Mineral based ATF starts to break down at around 220 degrees and is almost useless as a lubricant at 250 degrees.
On most Harley's the rear exhaust is in close proximity to the primary case and that adds heat too.
I have measured the temp on my primary after a hard run and have never seen temps above 180 degrees but that doesn't mean it doesn't happen. If you ride loaded, two up, in heavy traffic, the desert, or alot of hard pulls in the mountains then you'll probably see temps over 200 degrees.
Most people notice that shifting is immediately improved when they change over to ATF in the primary. This is because ATF is designed
for wet clutches. When you pull the clutch in it releases cleaner than with motor oil. You'll also notice that finding neutral is no longer a chore.
Personally I use any major brand of regular Dexron III and change it out every 5000 miles but if I did alot of the riding described above I would probably use synthetic (although I am a cheap old bastard).
I'm not promoting the use of ATF in the primary!
ATF has proven itself in hundreds of millions of auto applications and tens of millions of manual tranmissions and transfer cases.
You almost never see a 4wd transfer case failure anymore and I've personally seen 4wd trucks with a quarter million miles on a manual transmission and transfer case using Dexron III and still going strong.
I feel comfortable using it in my primary and I love not hearing the rattle when I snick it into first. But what I really love is not fussing with trying to find neutral on a cold day. And the over 300,000 miles I've put on primary's with ATF without a single lube related failure doesn't hurt either.