I have to agree with Tom & Richard on the lower thermostats. It is easy to fall into the thought process of lower thermostats will make it run cooler because of over heating issues, but it won't help with that. The thermostats 160 or 180 will be wide open (max flow) long before you start to overheat. The major down side to using to low a thermostat is just what Tom says. The engine performance and emissions will suffer during the times it doesn't have enough load on it to build the proper heat for efficent fuel burn.
I will have to look through all my bulletins, But I believe that DD sent one discontinuing using 160's in anything over the road.
These engines will run their best around 197 degrees