Amsoil was among the first commercially-available synthetic oils. It's always been good stuff. Fairly expensive, though--like pretty-much all specialty products from a small-volume manufacturer.
Short story, they've got three main "grades" of synthetic oil--"good", "better", "best". I use the middle one. They've got "hot-rod" oil with lots of ZDDP. they've got motorcycle oil, transmission fluid, at least two grades of gear lube, air-compressor oil, two-stroke oil...and generally several viscosities of each product. They've branched-out into additives and cooling system products, too. They also sell bypass filters, although all of mine are another brand (Frantz).
https://www.amsoil.com/c/products/1/
I buy NAPA synthetic oil, and ATF (by the case, only when it's on sale) for vehicles that I'm less invested in--stuff with worn engines, lawn mowers, cars owned by my friends that I get "volunteered" to work on, etc.
Far as I know, NAPA oil is produced by Valvoline.
Nothing wrong with it.
Drive to any repair shop that services big Diesel engines--the local Cat dealer, the local John Deere shop, the local Detroit Diesel or Cummins shop--and buy an oil analysis sample kit.
The kit is a sample bottle packaged inside a shipping bottle, with a pre-printed address label for the lab that does the analysis. I bought a case of 12 from the Detroit Diesel place; before that I bought them two or three at a time from the Cat dealer.
Or get them off of Amazon. Search for "Oil Sample Kit". There'll be plenty to choose from. Any listed for Aviation will be more expensive because guys with airplanes have lots of money.
Or just head to NAPA.
https://www.napaonline.com/en/p/FIL4077
You collect and mail the sample, they mail you a paper report of what they found--generally color-coded so you know right away what's "good" and what's "failed" (if there's any failure.)
I'm thinking they contact you by 'phone if the sample has problems, so you can change oil right away. Otherwise you wait awhile for the report.