Slybones,
First off, I try to keep the TC in the "locked" position when I am pulling. Especialy up a hill. I have a switch that I can lock the TCC, and keep it locked. This will certainly help in keeping the temp lower in the transmission by a large margin.
The gauge I had was incremented in 40 degree increments, so it was difficult to tell small changes in temps. However in all the time that I was doing the testing, I never exceeded the 200 F. degree mark. This was pulling GVW of 16,000 lbs. up grades as steep as 7 percent in hot summer temperatures. Now if the TCC is "unlocked", the temp will climb very rapidly, which is why I try to keep mine locked as much as possible.
According to GM's testing of ATF life, for every 20 degree temperature over 175 F the lifespan is cut in half. Varnishes will start to form at 212 F, Seals harden at 235 F. Plates will start slipping at 255 F. THe oil forms carbon at 295 F. and anything above 300 F will start the distortion of component parts.
The GM study was done using petrolium ATF. I was using a synthetic ATF,which has a much better oxidation stability as well as lower pour points which will allow the synthetic to function over a broader range of temeratures. In another test by GM, called THOT, which stands for "Turbo Hydromatic Oxidation Test", they require the (TAN) Total Acid Number increase not to exceed 8 in 300 hours. The ATF I am using goes to 900 Plus hours before it reaches a TAN of 8.
After the testing was completed, I sent a sample of the ATF to an oil analysis lab for testing, and the report came back as expected, "Normal", ATF is still servicable.
Wayne