Random thoughts.

From TCI:
Should I use an external transmission cooler in conjunction with the oil cooler supplied in the radiator?

Answer: Unless operating in an environment where the outside temperature is below 0°F, you should cap off the radiator cooler line openings and run your cooler lines directly to a new cooler mounted in front of the radiator. This allows the transmission to have its own cooling system and doesn't allow the engine water temperature to heat the fluid.
I've always believed that the trans cooler should be after the radiator. If the engine coolant gets too hot, and heats up the trans fluid (remember, the radiator will heat OR cool the fluid), the aux cooler is in place to bring it back down. If you run it through the aux first, and bring the temp down somewhat, a hot radiator is going to add heat.

BTW< Tech, If the fluid is too cold, it is too viscous vice not viscous enough. Higher viscosity = thicker.