I discovered Amsoil after Castrol reneged on a rebate. i wrote them back to complain (this was before the web), and instead of honoring their rebate, they sent me a John Force T-shirt where the John Force iron-on was about 6" off center. Classy.
I swore off Castrol after that. Discovered Amsoil at the MN state fair. Became a "dealer" (never sold any, but gave some away), and am now a preferred customer 'cause i don't have to go to a store. i place an order, their prices are competitive, and it shows up at my door in a couple of days.
Amsoil convinced me of longer drain intervals. I have gone 25K miles in my 1992 volvo between oil changes, and it uses 1 qt. every 5 K miles (at 300K miles w/ no engine work). I have 500 hours on each of my KTM and GasGas 2 strokes with no bottom end issues, and essentially no ring / cylinder wear (measure ring end gap) using Amsoil 2-cycle oil.
Is Amsoil better than other oils? Probably not (i am lazy, not an amsoil prophet) - to me, this just shows how good oils have become. our new toyota tacoma comes with "toyota care" where they do the first two oil changes at 10K mile intervals. I looked at bob the oil guy, and they rated the amazon basics synthetic oil pretty highly, so i ordered some of that in case the new truck needs top-up oil. it's at 5K miles now, and the new truck hasn't used a drop. Maybe the amazon goes in when i have to do my own oil changes. i'll see if that engine goes 300K on amazon oil - i am curious (about that, and if i live that long.)