Often and cheap
Use Dexron3/MerconIII from Walmart, and change it every year or 10K whichever comes first.
The only difference between the Dexron/Mercon and the 'recommended' ATF3 or expensive ATF4 is that they have additives that make them slipperier.
Who wants slippery ATF?? Soccer Moms in Minivans, and LHS drivers. We want grabby sticky non-slippery ATF to make firm positve shifts. [to hold the torque from the Cummins]
There is a chance that the Dexron/Mercon will have a slight shudder or grabby shift, if you get this, use one quart of ATF3. I've never yet had any shudder from using this less expensive fluid, and the cost savings is enough that you can afford to change the ATF whenevery you want as often as you want.
Hope this helps, Greg L