To double up on what NIW said.....
OIL cools an engine quite a bit - think about your lawn mower - air cooled true, but a lot of cooling is done by the oil itself!!!!
A less viscous oil, and an oil that does not break down over time will allow the parts to slip/slide their way without building up heat as much as another "type" of oil.
On hst machines - the oil IS the only coolant that the internal pump has - and running a "better" made oil .....will reduce the heat, sometimes tremendously.
Now to rebut this:
Fuel economy from an oil, especially that of the same weight, once again doesn't make sense. Fuel quality, ambient temperature and driving style play a huge role in this.
At the end of the day, I am betting your Kubota will last just as long with Kubota 15w40 and Rotella T6 as it would with Amsoil. Change it on time and keep the air filter clean and/ or replaced accordingly. The only difference you'll see with Amsoil use is a lighter wallet.
Ever notice that ALL European and German car companies are using 0w-20, 0w-40 oil ????????? SIMPLE.......pure and simple! IT GETS BETTER GAS MILEAGE - so yes.......a "thinner - better oil" will get better gas mieage (if that is what you are wishing) than the standard 10w-30 stuff!
As for wear .......IF you change your oil, any type - on a "regular" interval - you may never see or feel any changes in the condition of the internal parts..... however ......TAKE a tractor, car engine, truck engine - any internal combustion engine you wish........place "k" oil in it or any oil you wish, and run it 200,000 miles (change the oil as you wish) at the end of those miles......
the one that had AMSOIL in it.....will have less wear!!!! PERIOD.
and as for cost - it really is not "THAT" much more, as I said before - you can get a gallon of T6 ...and yes I use it !!! OR a gallon of Amsoil oil for about 5-10$ more......whether it is worth it, is determine on how soon you MUST change your oil!
Sometimes in life - rather than argue about doing something that you think is not worth it ......is to try it and say ...........mmmmmmmmm.........
remember that old commercial - "try it, you'll like it!"