Use TranSynd or one of the other fluids approved under Allison's TES-295 specification. ...
PS: I wrote the Allison TES-389 and TES-295 specifications.
Yes, I ended up getting the Transynd. Someone linked me to your separate Q&A thread and I got convinced. Thanks.
Found Transynd for $38/gal locally -- again, another link from a fellow iRV2-er.