Originally Posted By: buster
Regular Mobil 1 5w30 is like Amsoil XL 5w30.
Eh...I'd have to disagree. XL is 100% group 3, which is more like Mobil Super Synthetic. Mobil 1, is most likely a combination of base oils...not just group 3 (though I'm sure it does contain a large quantity of group 3...depending on viscosity). Mobil 1 5w-30, 0w-20, 0w-40, 5w-40 (TDT), and 5w-50 are probably the best M1 grades based on high quality (group 4/5) oil content. The Mobil 1 HM oils are also very good in this regard...probably.
But in reality, nobody really knows the full base oil breakdown of M1. Its probably changing on a regular basis too.