Why did you swap to a colder thermostat?
It's a 2016 RAM 1500 BigHorn V8 5.7 4x4, manufactured in October 2015.
Well, FAC/RAM really messed up the grille shutter programming logic on this truck, at least in this model year. The computer would close the shutters on the highway even during grueling hot summer days, even when towing several thousand pounds, even with TOW/HAUL mode activated. It didn't care. When I saw that, I immediately went to Mobil 1 FS 0W-40. That's a bandaid though, not a solution. When you have coolant and oil temperatures upwards of 250F while towing only 2000~3000lbs, you start thinking how you can keep the engine running reliably long term. People with EcoDiesels from he same same era suffered from the same problems.
Swapping the thermostat wasn't enough, as that only lowered the temperatures 15~20F. The real trick was to delete the grille shutters without the computer knowing so the radiators could get fresh air all the time. Once I did that, all my problems went away. All I have to do now to keep good fuel efficiency is run a 0W-20 or 5W-20, and that's it.
Now when I tow 3000lbs in the summer, I get maybe up to 210F coolant temperature, and 220F if I'm driving in the mountains going up a steep grade. That would be around 270F without this mod, with a similar RAM 1500 truck of the same vintage.
Also, mine didn't have an oil heat exchanger. Actually that one helps warm up the oil in winter, but when you have runaway oil temperatures, it can't really cool anything down.
This is how bad it can get with a 5th gen RAM 1500/Hemi, and these have been updated: