The stuff about not changing old fluid is an old wive's tale, IMO. Might have mattered 50 years ago, I don't know.
As for the type of fluid, back when I had a 3650, I did Mercon, Mobil 1, and Pennzoil Syncromesh all back-to-back-to-back. The Syncromesh was the best, hands down.
Tremec only...