I have been guessing that we haven't heard much about problems with DIT engines using A3/B4 oils because of the higher ZDDP levels, as that additive is a strong LSPI quencher...but that it is only a guess.
Are all the A3/B4 oils low Calcium (-1100) and high Magnesium (+500)?
I have seen a number of VOAs for M1 0W40 FS A3/B4 and they have all been very high in calcium (maybe 2800 ppm) and had little magnesium. I believe the M1 0W40 formulation before that was similar.
I think Castrol 0W40 A3/B4 is quite a bit lower than M1 in calcium but I'm not completely sure of that.
There was a summary of oil additive effects on LSPI from one of the additive makers (Infineum??) that showed a plot with the occurrence of LSPI really plummeting as the ZDDP concentration was swept not far beyond the typical SN/GF5 levels...I actually bought a bottle of Rislone ZDDP additive and was adding a bit at every change before d1G2 came along! Yes, I know I'm ridiculous...
Yes, I recall those plots, and I think it was Infineum. They showed that LSPI events increased linearly with increasing Ca add, hence the SN+ and D1G2 move to replace some Ca with Mg detergent. To decrease Ca levels for the same final TBN.
However they also showed that LSPI events decreased exponentially with increasing ZDDP, it was a powerful quencher of LSPI.
Most Euro A3/B4 oils that I know still have a full Ca detergent package with no or little Mg. However A3/B4 oils don’t have a restriction on their upper Phos (and therefore ZDDP) levels, while ILSAC-SN oils do have an upper limit imposed. Most SN / SN+ and GF-5 oils have about 700 ppm ZDDP while a typical A3/B4 Euro oil will have about 1000 ppm ZDDP. Recall a non-GF5 viscosity grade, like 5W40 has no restriction and so can be SN & A3/B4, it’s the thinner grades that are restricted.
I think when Europe was running full-SAPS A3/B4 oils with high ZDDP levels, they had no LSPI issues even with full Ca detergent packages. But recently they have moved to mid-SAPS C3 oils (like Dexos2) for both petrol (gas) and Diesel engines. This is probably where the problem with LSPI in Europe started as a C3 oil is restricted in the upper level of Sulphated Ash (SA%) permitted, and given that ZDDP adds to the SA% they tend to make Euro C3 oils with the same ZDDP levels as American GF-5 oils, but without swapping out some Ca for Mg and hence the start of seeing LSPI in Europe.