I have used both Amsoil and Mobil 1, and both are fine products. I am also an Amsoil dealer, but I just use it myself and sell it to friends at cost.
If you are getting Amsoil for less than Mobil 1, then you are probably using the XL 7500 series of oils. This is the one oil I do not recommmend: it is not the same sort of synthetic oil as the PAO-type oils, and I dont think it is as good as Mobil 1. It is a very good oil, as good or better than any dyno-juice oil, but it has a far different formulation than the more expensive oils: It belongs to the same family as Castrol Syntec (is it still called that?) which is not a very good synthetic oil.
I would strongly suggest moving up to better Amsoil oils that surpass M1 in quality like the Series 2000 or Series 3000 oils, or the basic Synthetic High Performance oils. The XL 7500 is geared to be the lowest cost synthetic oil targeted towards quick-lube shops and cost concious consumers.
Additionally, I routinely do 10k oil changes on most of my vehicles, and everything uses Amsoil: the car, boat (454-420hp), Suburban 454, motorcycle, lawn tractor and even the Waverunner (2 stroke syn oil). I also do oil analysis on most of them, and they normally test out perfect. My cars and trucks have over 100k miles on the original engines. I had a 1985 Camaro with a TPI V8 that was the highest mileage car I ever owned, and it went 180k miles on a combination of M1 and Amsoil; I sold it to a friend and it still ran strong and did not burn oil. My sister has a 94 Civic past 200k miles on Amsoil using 10k oil changes.
That said, you cant go wrong using Amsoil or M1 and a good filter.
