Just to let you know and not to argue, I teach thermodynamics at the university level and I am not trying to impress anyone with my background. Just trying to put out facts and not myths. Yes the heat is transferred through the cooling fins and not only the fins but all the solid parts of the engine, primary and transmission, as stated in the equation Q-UAdeltaT. The U is the property of the metal or other solid and its ability to transfer heat and the A is the area of the material to transfer the heat. But, unless you change the jugs or something, this remains constant. It is actually why the jugs have fins to increase the heat transfer surface area. Also very slight changes in friction can cause significant heat reduction. Since HD engines are oil/air cooled, the oil can have an effect on engine temperatures. The reason an oil cooler works is the increase in surface area along with a slight increase in the amount of oil in the system. I can also tell you that a synthetic is always going to be better for an engine. Dino can be fine but it needs to be monitored for breakdown and will usually not go as many miles as synthetic. The other quality of synthetic is its ability to withstand more heat with less evaporation and less breakdown. Dino is fine, if changed at the correct interval, synthetic is better and will handle the heat better. Use a good oil and change at the correct interval and you will be fine, but different fluids/oil will change the oil/engine temperature. Same principle as products like water wetter for water cooled engines and other additives that change the properties of the water in the system. I don't care whether people use Dino or Synthetic, they both will work, but I will always use synthetic in the engine and tranny. And different oils will cause changes in engine temp, probably not 40 degrees, but 5 - 10 degrees would be within the realm of reasonable changes. So as you said, use whatever makes you smile.