Is it possible to use calcium & magnesium levels from an oil analysis to guage what the remaining TBN of an oil is? (without having the TBN tested) Do calcium and magnesium deplete with TBN? Are there any other additives that relate to TBN?
No because it depends on the form (compound) the Ca and Mg are added in. Also, they are measured as the element and don't deplete as the TBN decreases (ie the base is consumed with created acids)