Wow! This thread got ugly. But it's kinda par for the course on any automotive forum as well.
Only 2 reasons I am posting this is to say on a water cooled engine, yes the thermostat controls the temperature. But it is the COOLANT temperature that it regulates. Since coolant does not circulate into the crankcase, it is the oils job to cool the rotating assembly (crank and pistons). Thus the reason for oil coolers in some applications.
#2 I did my own big test on Amsoil in a Mitsubishi Montero I had years back. For 3000mi, I used Valvoline Dura-blend 5W30. Used the same pump at the same gas station for almost all fill-ups. No other changes were made during this time period of 6000mi. The first 3000mi section had 2 tanks that were pure highway. Otherwise, just back and forth to work...daily driving. I averaged 19.2mpg over that 3000mi.
Changed the engine, trans and rear diff over to Amsoil products. Covered another 3000mi, and tallied an average of 20.5mpg This 3Kmi stretch did not have all highway tanks involved. Just daily driving. Now, I don't believe the engine oil by itself will make much, if any, noticeable improvement in mileage. But when that vehicle was traded in, the dealer appraiser thought it stalled while stopped because of how smooth it felt. So he proceeded to make that lovely starter grind noise.
That Montero and a Nissan I had were my guinea pigs to see for myself if that stuff made a difference. And in my experience, it has.
I will absolutely not condemn the idea of use whatever and change it regularly. Because it works just fine. I run an auto repair shop, and know that is the case. We all have our personal preferences, and sometimes the emotions of those preferences interferes with what our fingers end up trying to translate onto the keyboard. As well as, how what is written is perceived by the end reader.
For the Wing, I did not notice ANY difference with Amsoil 10W40 vs the PO's Rotella. Although I did not use an infrared thermometer on the differential before and after changing, I would bet that would probably show a difference there though.
So just keep smiling and enjoy the ride