Actually Steve, mixing oils is not so uncommon very seldom would you find a problem with additive clash as you know it. Where the problem exists is that one oil has it blended in such a way that it can go further than some other sub grade oil, but when mixing different brands of oils, it will then create an unbalance of the additives therefore, you upset this balance and it may not perform as it was designed, thus the clash.
consider like the additive problem shown on the front page of bobistheoilguy, where a good gear oil, blended with a special amount of antifoamant additive, then you take another oil, which is what lucas is, and put it into the blend, it will be compatible, and mix but it unbalanced the additives, thus makeing the original balance not work with the mix, causing excessive foaming.