My coach is equipped with an amber trans fluid temp warning light which, I believe, is supposed to come on @285F at the retarder output. I've seen it a few times. I try to limit my trans-synd temps to 250F @ retarder output which usually involves no more than level 3 in 5th or level 4 in 4th for any extended downhill run but I usually gross about 65-70,000#. I use the brakes to keep rpm's under 2000. My last trans-synd change at about 25,000 miles use came out pretty good; I could go a little longer. Obviously, this is severe use. I'm on the original trans @150,000 miles.