Hi All, new here. Any advice on Mobil 1 vs Amsoil. New mechanic for my 5.3L 2008 Tahoe LTZ recommends Amsoil. He says it's the original and still the best. Currently using Mobil 1. Also, my sister's mechanic replaced synthetic oil with petro-based oil in her 1999 Saab (160k mi, 4 cyl). Is it kosher to go back once synthetic being used? Thanks.