The behavior that you’re describing is normal on these rigs in my opinion. I have both a 2007 and a 2012 Denali, and pull a 3000 pound trailer often. On a hot day, going uphill at low RPM, my temperature gauge creeps up to between the 210 mark and the one above it. Happens on both rigs. I’ve read a lot about towing with these things, and others have said the same. The only thing that keeps these things cool pulling a trailer in hot weather is an aftermarket transmission cooler. Seems to help both transmission and coolant temperatures.
Keep in mind that the boiling point of a 50/50 mix of antifreeze and water at 12 psi cooling system pressure, will be 259°F so there is plenty of margin built into the cooling system even if the temperature creeps up over the 230 mark.
Also, 5w-30 is what you want, especially pulling in hot weather.
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