IMHO = In My Honest Opinion
The transmission is designed to operate most efficiently at 160-200 degrees. Below this, the fluid viscosity changes and may affect the life of the transmission, although I couldn't say for certain. I do know that as far as the PCM is concerned, anything below 100-120 degrees is still considered "cold" and the shifting strategies change slightly.
If the fluid isn't staying up, I'd almost consider bypassing the additional cooler for the time being until the ambients warm up.
Your best bet? Ask a tranny professional what is too cold. Brian's Truck Shop, Suncoast, or any of a number of other professionals would be able to better advise you on this.
Take care.
__________________
Bill Cohron - The Mad Doctor  
Power Hungry Performance - The ORIGINAL in Ford performance tuning... Since 1996!
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