Jim, you probably didn't get $3 or $4 worth of extra benefit from mixing the Mobil 1 into your other oil. Oil company chemists develop complete additive packages that do the best job they can for the price point they're told to hit. Mixing additive packages, while not a big problem, probably means that none of the oil works as well as it could if it wasn't mixed. Even with sticking with the same brand, Mobil for example, there's no reason to think that the add pack in Mobil 1 will improve the different add pack in Mobil DriveClean.
Ken