[svxistentialist]

Doug,
A thermostat will certainly help, it will stop the oil from being cooled too much, and then release oil to the cooler once past a certain temp.

However, in colder weather, this will set up a "cycle". Once the cooler is open, it may have the ability to cool the tranny below lock-up temp again. My suggestion is simpler, use cable ties or wire to blank off a portion of the cooler fins using cardboard or something which will not soften for rain[assuming it is out front] Start with one third blanked, and go to a half if it does not lock up quickly enough. Get it right[one third maybe?] and you can leave it on all year round and forget about it.

Joe