Well the fact that most of their tests are direct comparisons against Royal Purple tell you that's who they consider their major competitor. Also, could be slightly biased since they only show you the tests where they come out on top.

I know in the Lotus forum there have been some pretty hot debates on oils... but I've never heard any of those members suggest Amsoil. Most go with Royal Purple it seems. This coming from autoX and track racers who do push their cars to high temp situations.