What MK was saying, IMO, is that you can achieve similar results using both Mg and Ca, but the new overbased Ca is a more expensive approach. It's the balance of the two that you have to reach. In his approach, he would use the more expensive Ca. Mg is rarely used anymore and it's more of a cost issue among most blenders.
Most CI-4+ or ACEA "B4" rated diesel oils will have the high Ca and low Mg, to minimize high temp carbon/varnish deposits on the crowns, the lands and in the ring grooves.
This really isn't an issue with water cooled, gas engines, but it is with air cooled motorcycles and other smaller, air cooled four strokes, since they run so hot....
Most CI-4+ or ACEA "B4" rated diesel oils will have the high Ca and low Mg, to minimize high temp carbon/varnish deposits on the crowns, the lands and in the ring grooves. ....
If you take a look at VOAs of Mobil 1, you'll note that the bulk have relatively high calcium and low magnesium - except for the SUV (Delvac 1) which has a greater ratio of magnesium.
The additive which shows up as magnesium has some advantages in prolonged drain intervals in diesels for which Delvac 1 was developed.
The downside of it is that it contributes to spark plug fouling in spark ignition engines.
We will just have to hope that with the change in formula has come an equivalent improvement in the suppression of the negative effects of the Mg (if that is possible through chemistry). We all have to keep in mind that as technology changes, there may be an accompanying change to the way we have normally seen things work.