Get as large a tranny cooler as you can, keep the fluid out of the radiator, and run the regular Hy Gard. I even insulate the tranny lines next to the engine to limit the amount of heat that they might otherwise pick up there. The cooler than optimum temps will not do anything bad to our trannies. We just don't drive them enough to. That came from Lenny and is the way I have driven my cars for years now. And the only reason to add any ATF is to give the fluid some color that makes it a little easier to read on the dip stick. Otherwise it is not necessary. I don't add ATF, I just put some baby powder on the stick to read the level.

https://jdparts.deere.com/partsmkt/...draulic.htm#_Hy-Gard_and_low-viscosity_1