Thanks for the tip on trying a dealer. I generally seem to find dealer pricing high so hadn't checked with them. Just checked my closest Allison distributor in Sacramento, and they're price is $46.43/gallon. Their filter kit was also less expensive than I had been able to find online.
I'd like to use the method Gerald has written up to get more Transynd into the unit. Someone else had previously asked which line was the one to remove for draining, but I never saw an answer. However, on the bottom surface of the sump casting I see cast in features below the cooling lines. Looks like they may be indicating fluid flow direction, but I'd better make sure. I'm assuming the arrow head shape facing away from the sump is showing the line out to the cooler, and the other is an incoming "V" shape for the incoming line from the cooler. Is that correct, so I'd remove the incoming "V" line to drain the old Dexron until I see the new Transynd running clear?