Hopefully, one of our experts chimes in. However, if this were a quiz show question and I were pressed to answer, I'd venture a guess that the magnesium based detergents are a better fit for getting the high TBN and low SA required by certain ACEA specs that the big name HDEOs meet. I would gather that having a TBN of 10 or higher along with SA of <=1 isn't easy, and then there is one ACEA HDEO spec that some meet that calls for low phosphorous, too, in addition to high TBN and low SA.