Actually Amsoil has been third party tested many times, and the conclusion is the same, one of the best synthetic oils money can buy. Just check a VOA on Amsoil and you will see compared to others. Mobil1 is great too, and would be my second pick if Amsoil was not easy for me to get. Some of the synthetic oils are crap and so are some of the motorcycle specific dyno oils. I choose Amsoil first because of product performance, second because they will stand behind there product if there is a problem...you can actually talk to a real person for questions if needed...try that with one of the big boys.
Both are great and really down to your choice and accessibility.