[BigAls87Z28]

I was reading that little FAQ about GC...
Ok, so its a 0W30 weight only as its the only sythetic castrol oil that is actually made using Group IV base stock.
Mobil 1 does use Group IV stock. Not sure on Valvoline's sythetic stuff. Im gunna have to break out my valvoline book and see what the base stock is.

5w30 of Autozone oil is has the same weight at cold(first number) and opt temps (last number) as 5w30 Mobil 1 or Amsoil 5w30. Its what the oil is made up of that changes.
Sytheic, weither using PAO base or a moddified Group III base stock, will protect better then regular base stock oil. It operational temperature range is much larger then non-sythetic as well as several other things to help keep the engine better protected.