Amsoil is not god of all oil, just because they did the testing and publish the result from their labs, doesnt mean anything. Pennzoil and mobil 1 have good results after Used Oil Analysis - tells you bunch of things about practical usage - please check Bitog.
Its just like pharmaceutical companies, they can publish whatever lab result they want, but they still need to collect data back from the public testing, because alot of drugs works wonders in the lab only to fail in real life - side effects, etc.