Hi Woodjointer,
I moved your topic on its own because it is a good question.
If you are getting transmission fluid mixed in with the oil, I wonder if that is maybe burning a little hotter? I am guessing the rotella is the 15w-40 version which I wouldnt think is much different than any other motor oil, just a little heavier. I would guess the other things mixed in with the stuff from your mechanic is what is raising the temp.
Anyone else have some ideas?