I agree the synthetics are better, as they can be manufactured with the great properties that make them superior, properties that are difficult to add to dino lubes to try and acheive the same results....but the high cost has always been a factor for me.
Longer drain intervals are usually necessary to help "sell" synthetic oil and I can see that for gasoline engines. Or any high performance hot running application such as racing where premium engine oil is required and the cost doesn't matter.
Diesel engines are "dirtier" than gasoline engines as the very high compression ratios for compression ignition result in more blow by and the resulting contamination of crankcase oil.
But for us "run of the mill" types that want to do the right thing for our tractors but have to live within a budget, I feel following the Op Man is the best way to go.
I use Shell Rotella T 10W30 and change engine oil and filter every 100 hrs per Kubota's Op Man.
I like Amsoil and started using it in tiny engines....like my Honda 4 stroke brush cutter that sees a lot of hot and long duty at full throttle......and I'll change that engine oil once per year (season). Cost is not a factor since the amount used is so low.
As for the tractor that sees light, all year use (less than 200 hrs/yr) I just can't justify going to oil that is many times the cost of the dino.
My oil change costs $20 per with the Rotella, a brand I trust for quality.
That oil change would cost $88 with the Amsoil Series 3000 Synthetic 5W-30 Heavy Duty Diesel Oil.
If you doubled the interval to 200 hours (which I would never do)....the cost remains pretty high at $44 vs $20
So, while I can appreciate the value of synthetic oil, you sure can't do any better for your engine, and Amsoil is one of, or perhaps, THE best, I feel it is "over kill" in my case.
JoeK