While I haven't done dedicated comparisons, I have personally seen several benefits in the course of my switch to Amsoil synthetics over the years. I've read and seen enough elsewhere to convince me to switch and stay with it. My engine builder's recommendation was al major factor.
* An obvious operating temp. difference cruising down the highway when I went from Rotella 15-40 to Amsoil 15w-40 on a '92 Dodge diesel dually as well as a slight mileage increase. My 2004 mileage was slowly improving due to break-in before & after the switch to synthetics before 20.000, so I don't know how much was attributed to the oil. It leveled out around 30,000.
* While some of it may be a result of viscosity, when I switch from conventional 30w oil for break-in oil to 5w-20 Dominator race oil, I have to turn the idle down on the race car.
* My transmission builder has said to keep using whatever I'm using in my transmissions when I bring them in for freshening after several years even though he has his own recommendation.
* My differential guy has also said the same thing. Bearings are often reused when the ring & pinion are replaced.
* Less wheel bearing issues on my heavy open trailer and stayed with it on my enclosed trailers.
There are "different" synthetics to watch for and each manufacturer has their own recommendations. But racing is a harsh environment so I wouldn't suggest "extended" drain intervals. There is still some wear and fuel/combustion contamination. I run the IHRA circuit and several Stock/SS association and bracket races during a season and change the oil several times..
If you haven't seen it, this site may provide some interesting reading.
https://bobistheoilguy.com/