The manual transmission came from the factory with ATF. (I believe my 96 Viper came with ATF as well). The excellent Crossfire technician that work on my car in 08 drained the ATF and replaced it with synthetic motor oil.
I had purchased a couple different transmission "additives" to give him before he changed it, and he told me, "I'll put them in if you want me to, but nothing is more slippery than synthetic motor oil, and you will probably just be wasting your money". I figured he knew a lot more about it than I did so I just went with the motor oil.