I go by personal experience, #1, and testing results if available, #2. With Amsoil I've seen both - I've seen the testing, and I've seen engines with MANY hours I've torn down, both 4 and 2-stroke, that flat amazed me. I can confidently say in that case, you DO get what you pay for. I've also seen oil tests of Walmart branded oil, and most would be shocked to find that it knocked some other more well-known-for-decades name brands right off their perches. It's not as bad as you would think!
A sort-of related topic, there's a running joke about Walmart batteries: NEVERstart. Funny thing is, each and every single Everstart battery I've used, regardless of the application from big diesels to lawn tractors, I've got a minimum of 8 years out of. I've heard similar stories all across the net in automotive, truck, motorcycle, ATV / dirt bike, and OPE (Outdoor Power Equipment) forums as well. Truth be told, there's only 3 battery manufacturers in the North American market, so no matter what brand you buy, it was built by one of those 3.
Oil threads are funny, since there's a lot of strong opinions about lots of oils. But facts are facts, and there's no getting around that. Sure, you can run cheap oil, but you'll be scraping carbon and rebuilding engines much sooner than if you had run a top tier oil, especially when things get "out-of-spec", like a coolant leak on a liquid cooled engine, or cooling fins and fans that get blocked off my mouse nests and mud on a air cooled engine. When that happens, you can bet I'd want the expensive oil in that engine, 2 or 4-stroke, air or liquid cooled. The expensive oil will live, while the cheaper oil probably won't.
Remember, I make good money because of cheap oil.
Someone else's pic:
