Jaybird, we've got a 2008 GMC with the Duramax/Allison combo. We were towing a 3400 and now a 3725, both of which I would consider large. In our case, I follow the manufacturer's suggested maintenance for "average" as I don't tow all the time. Don't know if you have a gage/temp sensor on the transmission, but we do. I keep an eye on it to make sure it doesn't get too hot. 200 degrees is the highest I've ever gotten. 225 is starting to get "iffy" and if it starts getting up around 250 or so, breakdown of the oil starts occuring, with tranny failure a likelyhood down the road. Had it changed at 15,000 when "new" then again at 30,000, now just turned 65,000 and had it changed again. Everything looked good, and the tranny is running just fine. Probably won't do it again until it hits 100,000. But, it's all in what you are comfortable with.
