I've changed the fluid and filter every 30k, with Castrol ATF4. I don't tow all the time, but I do occasionally tow a car/trailer or a ~5k lb equipment trailer. It has a shift kit in the valve body, 3.92 final drive, and an extra cooler before the radiator cooler. Truck was at 142k with the next change due at 150k, but it started acting up a little. It would pulse a bit in 1st when cold. When warm, it would pulse when coming to a quick stop, and had even choked down a couple times. I could let it roll a foot and re-apply brakes and it would settle down. It was like it couldn't decide if it wanted to let go or keep going. I considered just changing fluid and filter early, but it was really clean and didn't show any varnish.
I took it to the locally recommended transmission shop. He drove it around and suggested replacing the solenoid pack. It's a 6-solenoid assembly that sits on top of the valve body and controls shifting. He did that, the external line pressure solenoid, fluid and both filters. $950 hurt my feelings, but it smoothed things out and it's working fine now. He said the solenoid pack/wiring harness get dicey with age (it's an '06)