My Girlfriends 07 Tahoe Z71 was neglected service wise in the earlier parts of its life. Of course she has no idea if the transmission has ever been touched. Has 195k but runs reasonably well. Trans fluid is dark and smells well used but not necessarily burnt. I decided to suck 3 quarts out of the dipstick hole and replace with some Supertech dex 6. Still feels normal so far. Rear end assembly was replaced before 100k. No known services with that either. Sucked that fluid out and threw some Valvoline Full Syn 75w90 in it. Was debating 75w140. What are your thoughts on going thicker?