I typically run Amsoil in about everything I own. There are very few instances where I and my brother don't use their products. For instance, I have a Silverado with the auto trak transfer case where I run GM fluid, my brother has a 370Z and he runs a Redline product in the transmission (the engine and rear axle run Amsoil). The reason for the Silverado running GM fluid is because Amsoil doesn't provide an equivalent fluid, with the Z, Redline has a fluid that is specifically meant for the Z's transmission and it works very well.
I'm particularly picky about equipment and generally research things to death before making a decision on something. I can honestly tell you I have spent hours researching different oils. Most manufacturers provide technical data on fluids where certain ASTM test have been performed so it's pretty easy to decide what is best. Sometimes you do have to dig a bit to find the data you're looking for but it is out there. This is the sort of data that backs up Z71's claims and for what I've seen Amsoil generally comes out on top followed very closely (or sometimes equal) by Redline and Royal Purple.
Diesel oils is one area where the three manufacturers above really stand out. Since 2007 the government has mandated that most diesels be produced with particulate filters and require ultra low sulfur diesel fuel. Diesel oil had to make a large change with a large reduction in certain anti wear additives to prevent damage to particulate filters. This is the new CJ oil rating you see on most diesel oils.
If you have a diesel without a particulate filter try to run CI or CI4+ rated oil in your engine. CJ rated oil is meant for diesels with a particulate filter and just isn't of the same quality as CI or CI4+ oil. I have a Kubota tractor and the owners manual flat out says "CJ rated oils must not be used in this engine". If you compare the wear ratings, additive package and TBN values of CJ compared to everything else, you'll see that a CJ rated oil is of no comparison to CI and CI4+ oil.
It's sad to say but the good CI or CI4+ rated Rotella, Delo, JD Plus50, ect are not available anymore. (As an aside, the JD Plus50 was one of the very best oils ever made, it had TBN values that were higher than anything offered by any other manufacturer, Amsoil included, all in a non synthetic oil!!!)
I've ran the Polaris recommended diesel oil since it is CI rated and have also ran a couple changes of Kubota CI rated oil since my hours are still low. Once thats used up I'm moving to Amsoil's CI4+ oil. I run it in my Cummins and it performs great. A lot of people think its overkill, but I've seen excellent results in running Amsoil. One example among a few I can provide, I run Amsoil in a JD lawnmower with a Kawasaki air cooled engine with about 1200 hrs on it. Before switching to Amsoil you would need to top off the oil before every other mow. Since the change there is no need to top off, the Amsoil just doesn't burn off. I do one change a year with around 100 hrs operation per year. Same thing with the Silverado, it has just under 130000 miles. I bought the truck off my dad in 2007 and ran what he used (Pennzoil) for a while and would need to add about a quart in 5000 to 6000 miles, I switched to Mobil 1 and ran it for a while, same result, I'd have to add a quart over the life of the change. I finally changed to Amsoil and no longer have to top off during the change. There is some loss, but it's not enough to worry about.
One other Amsoil story involving the automatic transmission in the Silverado. When I got the truck the transmission shifted pretty slugishly and just felt like a dog. I did a fluid change and since it was a higher mileage truck over 75000 miles, I ran the Valvoline high mileage Dexron fluid with the same results. I finally changed all that out, added a deep mag hytec pan and put in Amsoil. The difference is like night and day! It improved shift quality and firmness as well as acceleration!
For what its worth, I don't make a dime from Amsoil, I'm not a dealer and have no interest in being one, I just use it.