If you read a lot of the literature on multi viscosity oils, they will tell you to use a 10W-30 over a 10W-40 given the fact that so much of the "
oil" is lost due to the amount of additional additives that must be used to get to the higher number (40).
In non
synthetic oils, I would have to agree with Tom T that single viscosity oils make more sense that multi. The only reason for the manufacture of multis is to assist in cold weather starting in the northern part of the country.
Mobil 1, as a
synthetic, is such a superior product that I don't know why one would use any other type of syn. It is required by many manufacturers (GM/Corvette) for extreme use situations.