Final Update:
After driving with the Allison Transynd in my A413 for several months, I have definite proof it's not compatible. Here's the data in the order of modification:
Overall gear ratio: 3.22
Fluid: Dexron III
MPG: 25
Trans temp on freeway: 130
Overall gear ratio: 3.02
Fluid: Transynd
MPG: 18

Trans temp on freeway: 190
Overall gear ratio: 2.6
Fluid: Transynd
MPG: 22
Trans temp on freeway: 190
Overall gear ratio: 3.02
Fluid: Dexron III
MPG: 25

Trans temp on freeway: 140:thumb:
Note how the temperature is still slightly higher after I flushed out the Transynd and put Dexron III back in. There must be a small amount remaining. I'll flush it again for good measure.
I don't have exact knowledge as to what caused this drop in MPG (performance too!) and increased heat, but I feel the torque converter was the culprit as it may have been incompatible with the Transynd. Maybe Transynd requires a different TC design?
Well, this was a failed experiment, but what's life without adventure? A good friend of mine who raced NHRA Comp Eliminator said that in experiments like this, 90% is research & development. Only 10% of the parts, fluids, & procedures tested are actually used to race.
I hope everyone can get some useful information from this. I have more MPG experiments lined up. Stay tuned.