Like charlie said, dexron 3 was the standard fill ages ago.
I recommend sticking with it. It's good enough for the job and you can get it from almost anywhere (parts store, hardware, grocery, gas station, etc.)
Transynd is a great fluid. It's a premium synthetic which will last many many miles and run cooler doing it. But for most, it's not necessary and just wasted money.
Some people have used John Deere hy-gard hydraulic fluid. I've never done it so I can't recommend it.
Some districts even used engine oil with allison's blessing