Running cold will mostly hurt your mpg, the extra wear on the tranny is very minimal, close to being negligible. You only need to do someting if it never gets up to operating temps.

 

Back when I had a real job I managed a small fleet (among other things) and we had a 460/C6 that the station manager insisted on running the factory stock 3.08 ratio (bought used) instead of the 4.10's I had supplied to use for heavy towing. They fried a tranny and the outfit that replaced it installed a LARGE cooler as the answer rather than insisting they use the 4.10 gears. It seemed to work until they were up here in the north country running the Interstate during a major sub-zero (-15* to -20*) cold snap and again fried the transmission due to over cooling. The fluid got so cold it "jelled" (or so I was told by a Ford service manager) in the cooler and blocked the flow.

Based on this experience, IMHO, yes, you can overcool a transmission.

I saw tranny temps (test port mount) ~220-225* towing in 95-102* ambient temps ... upgrade with a head/crosswind as usual ... on our trip this year. Since we usually do this southern Colorado trip (across Nebraska and NW Kansas) in July I'm adding an additional cooler in series with the stock TT unit before our next trip. I want to drop those temps at least 20* even though I service (Wynn's Flush & Fill) the tranny every couple years. If I use the truck in the winter (unlikely since Clyde is "babied" that time of year) I intend to throw a windproof cover over the front of the secondary.

Roger

Clyde S Dale, my chestnut brown & tan 5th wheel workhorse is a
1984 F250HD XLT RCLB 4X2 8600 GVW
460 C6 3.55's
67,500 original miles (and counting)