The two commonest (aka cheapest!) forms of overbased detergent additive used in PCMO are 300 TBN calcium sulphonate & 400 TBN magnesium sulphonate. I did a quick bit of Googling and the former contains about 12% Calcium and the latter 9.2% Magnesium (note that only the pure metal part of the additive shows up on a UOA or VOA).
So to get a boost of 1 TBN, you need 0.33% of 300 TBN Calcium Sulphonate which is equivalent to 0.0396% (or 396 ppm) of Calcium metal. To get the same 1 TBN boost, you need 0.25% of 400 TBN Magnesium Sulphonate which is equivalent to 0.023% (or 230 ppm) of Magnesium metal.
So if an oil formulator, faced with passing an LSPI test, part replaces Calcium detergent with the same class of Magnesium detergent but still keeps the same overall oil TBN, then the overall content of metal in the oil will go down.
I never looked but I would not say that Magnesium has any better TBN retention properties than Calcium. What you want from a detergent is something that mops up acid (derived from the oxidation of base oil) as soon as said acid is formed. The whole concept of TBN 'retention' is IMO deeply flawed because it's a way of pretending that inefficient detergents (notably overbased salicylates) are 'good' when in reality they are 'bad'.
So to get a boost of 1 TBN, you need 0.33% of 300 TBN Calcium Sulphonate which is equivalent to 0.0396% (or 396 ppm) of Calcium metal. To get the same 1 TBN boost, you need 0.25% of 400 TBN Magnesium Sulphonate which is equivalent to 0.023% (or 230 ppm) of Magnesium metal.
So if an oil formulator, faced with passing an LSPI test, part replaces Calcium detergent with the same class of Magnesium detergent but still keeps the same overall oil TBN, then the overall content of metal in the oil will go down.
I never looked but I would not say that Magnesium has any better TBN retention properties than Calcium. What you want from a detergent is something that mops up acid (derived from the oxidation of base oil) as soon as said acid is formed. The whole concept of TBN 'retention' is IMO deeply flawed because it's a way of pretending that inefficient detergents (notably overbased salicylates) are 'good' when in reality they are 'bad'.
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