Transmission temperature max of 240 deg. sound right?

I don't know that synthetic oils run cooler, but they are more tolerant of higher temps. Using them is good advice. Temps in the range of 240 when pulling a long grade with a load (trailer) are not uncommon, but they should drop back to 200 or less on level ground. I went to a deeper oil pan to increase my oil volume (3 extra quarts) and a larger tranny cooler which has helped. Make sure your cooling stack is clean and your fan clutch is working properly.