Amsoil compared their 5W-30 oil with Mobil1 5W-30. Results are linked on Amsoil's product information page. Guess they wouldn't put it there if it didn't outperform the Mobil1.
A Camaro near here changed from Mobil1 to Amsoil and gained 11 HP on a dyno. I'm getting up to 19MPG in my 4WD T100 around town here pretty regularly using Amsoil 0W-30, which makes me pretty happy.