140* is too low. You'll get more wear in the cylinders when the temps are below 160*, compared to a warmer engine. Also,
oil temp tends to follow water temp. If your
oil is only 140*, it's not warm enough to do it's job.
The first thing to do is check the gauge for accuracy. With the engine cold, remove the radiater cap. Let the engine idle and slowly warm up. Use a probe thermometer to measure the temp and compare to the gauge.
If the gauge is accurate, then the thermostat is probably faulty. Don't jsut replace it. Take it out and test in a pan of water on the stove.
If all that checks out, then you have a cooling system that is excellent, and more than you really need. Not a bad thing - it's much easier to allow the temp to get higher than it is to bring it down.
On a cool day, cover 1/3 or so of the radiater with duct tape. Use black or clear so it doesn't show so much. But that works really well for me. I usually start at the bottom.