I can offer nothing in the way of what the chemical makeup of either oil is. I have, however, used both before and have some conclusions of my own. I feel the Amsoil is a tad expensive for a group III oil.....but I would say it's not WAY out of line given it's record of excellent analysis. I suppose the same can be said for the Mobil 1 (but the cost is probably lower). Nobody really knows for sure what Mobil 1 uses, in percentage form, as the base stocks. From what I've discovered, using both oils, is that the Mobil 1 DID cause my particular two engines MORE chatter and audible engine noise that the Amsoil XL did. This isn't to say that Mobil 1 is causing wear issues but regardless, that was what I experienced.