Newer vehicles don't allow the Trans TCC to engage/lockup until a certain temp is reached. The slippage in the TC quickly heats the fluid. Engine oil & Trans fluid needs to be hot to prevent premature wear & condensation buildup.
The Long/Tru-Cool type Coolers are self regulating, cool/thick fluid bypasses the Cooler thru the larger upper 2 rows allowing the Trans to quickly reach operating temp, hot/thin fluid flows thru the whole Cooler.
There are also Remote External bypasses available for other types/brands of Coolers.
Long/Tru-Cool flow diagram>> http://www.bakerprecision.com/trucool.htm
If you're not using a Rad w/built in cooler, use a "Large" auxiliary cooler and if you're using a TC with a Higher than stock Stall rating, use an "Extra Large" auxiliary cooler.
From PATC's website:
You must have a transmission cooler in your radiator regardless of what someone has told you for the transmission to last. Auxiliary coolers are just that, in addition to. Water cools 32 times better (faster) than air always, period. In the case of air vs. water, there is no contest - water is way better. The transmission fluid comes directly from the torque converter at a much higher temperature than the water in your radiator and is cooled to the water temperature fast. Then it goes to the auxiliary cooler to be cooled far below the water temperature. If you don't need a cooler in your radiator why does GM spend all that money doing so? If you wanted to cool a red hot piece of steel fast would you stick it in water or air, see the point. Your transmission will run cooler with a lock-up converter. This is more important with stall speeds of 2000 RPM or higher.
TCI's website, safe Trans Temp chart: