Any oil formulation is a trade-off situation.
Magnesium dispersants are effective at dispersing soot so the soot paricles do not agglomerate, but this looks like an old school formulation.
In a situation as the above, the calcium and magnesium are in a combined detergent/dispersant additive package. If the volatility of the total formulation above is low, then there will not be much ash deposited.
The HDD additive package I use has high calcium and no magnesium whatsoever, but has a very effective non-ash or sshless dispersant.