A friend of mine put in a $10 Stant thermostat 2 years ago and it still works flawlessly. Get one that is "OEM temp".

Look at the temperature needle. Is it at or close to the halfway mark? If it is past it, you're running too hot. If the other way, the engine is running too cold. And also, if it's running cold the needle will dip down closer to "C" at highway speeds.