I don't know where you've got your information to have the concerns you have. You have to assume whatever ATF is currently in the system is compatible with the original Dexron 3 spec. You can mix fluids as long as both were made to cover the Dexron 3 spec. Valvoline Maxlife is what I've been using for a while. I use to run a Dex/Merc. If you want to renew the whole system and start fresh, I have a video you can use.
Transmission Fluid Flush - YouTube