Dan,
Dexron fluid was originally used in your transmission, and if you are not sure if it has been changed to TranSynd, I would urge you to have a fluid analysis ran to determine the fluid type and condition.
There is a way to purge the Dexron fluid from the torque converter and transmission cooler when changing over to TranSynd, but it normally takes two people and can be a little messy. First you drain the transmission, change the filters and refill with TranSynd. Then you disconnect the return line from the transmission cooler to the transmission and place the end over a drain pan. Next have someone crank the engine, let it idle, and immediately kill it when the fluid flow starts to change color. This will get as much of the Dexron fluid out as changing the fluid twice does.
Gerald
PS: Allison no longer recognizes Dexron as an acceptable fluid for use in their transmissions.