I went ahead and changed the fluid with Mobil 1 75w-90. I didn't remove the cover, I just sucked it all out with an electronic portable suction unit off my ambulance (pretty creative) and refilled it. Seems to be a lot smoother, but it could just be my imagination. I never really paid attention, but is there a drain plug on the transmission? I won't have time to take a look for a few days. I was told my a chevy dealer not to worry about the filter until 100,000 miles and to just drain and refill. I've got a hair under 80,000 now. Is it okay to put synthetic in without getting all the original stuff out?