What if any difference is there in Mobil 1 and Amsoil. I'm looking at 5W-30 weight oils. I have a vehicle that is still under factory warranty and I'm concerned about the statment in the owners manual that says use only oils that have the starburst symbol for API certified oils. The Amsoil 5W-30 original synthetic doesnot have this symbol. If I have engine trouble I'm concerned that GM will use this to not honor the warranty. So, with this in mind I'm thinking of using Mobil 1. Is Mobil 1 as good as Amsoil?