transmission fluid temps if measured at the hot line (not the pan) are best if kept 180 degrees or less. Over 220 for sustained periods is very bad. Temps can easily hit 260 when backing a trailer, especially up hill. At that temp seals start to harden and fluid
life is only about 7000 miles. Those temps are OK for very brief periods. After backing a trailer and if my transmission temps are around 260 or so, I leave the engine running with trans in N till my temp gauge shows 180 before I shut it off, even if the EGT gauge is showing 300 way before that.