I have a 2009 Suburban. It has the six speed automatic.
Since I had nothing but trouble with the 4L60E in my older Yukon, I had the trans in the Suburban serviced by the shop I've been using for years immediately upon purchasing. The truck had 9,000 miles on it when I got it.
I did it again at around 55,000 and again at 115. At 115,000, the trans fluid was a little burnt. A few weeks ago, truck had around 139,000 miles, I checked the fluid before a long trip and it was burnt again. I went to the shop, the owner looked at it and suggested a flush and a service. I suggested adding an external cooler. I had the truck for a week prior to the trip and I monitored the trans temp and the condition of the fluid. It was fine. Temps were awesome. 147 on the highway doing 70 on a 74 degree day.
Drove down to Florida. Saw trans temps as high as 190 in bumper to bumper traffic in 99 degree weather. As soon as traffic started moving it would creep down to 178-184.
Strangest thing was on the way home, as soon as we crossed from Florida into Georgia, the air temp dropped ten degrees and the trans temp down along with it to around 168.
Fluid has been in good shape. I'm pretty happy. I just bought a house and really don't want to drop 3500 bucks on a rebuild at the moment.
When Pat (the trans shop owner) serviced the trans, he said the inside looked better than the fluid would have hinted. That's why I went ahead and took it on the trip.
I only wish I had ever monitored the temp before the cooler install.