Originally Posted By: kschachn
This also shows why it is misguided to think that an oil mixture can be evaluated via VOA. Take zinc for example, the VOA only shows the concentration of the zinc atom and doesn't tell you anything about the structure of the coordination compounds. Zinc dialkyldithiophosphates are a variety of compounds and unless you know which exact one (or ones) are being used how do you know that the resulting mixture is overall beneficial to the performance of the oil? You don't.
ICP or AA is like trying to evaluate words and letters on shredded documents. Sure you can tell whether the ink was black or blue or red, but trying to divine words and phrases is not possible. On the flip side there are only so many compounds added to oil containing zinc so perhaps you can take an educated guess, but it should be realized that you aren't measuring the zinc compounds via an elemental analysis at all. You really do not know anything about that via a standard UOA or VOA, only that there is zinc present. I could make up an oil sample using an oil soluble zinc compound that would exactly mirror the zinc concentration of some other oil but would be completely ineffective as a motor oil additive.
Technically, you are correct. However, the oil manufacturers list zinc in the same way on thier PI sheets. Now admittedly this is probably partially for marketing , but I think in general, you can compare zinc levels on PI's or UOA's, in that a reputable manufacturer is probably not going to use a zinc compound that shows up in high PPM's in a UOA or PI, but is ineffective. As an alternate view, let's say when it comes to calcium, I don't care what compound it is, as it appears in LSPI testing, any calcium compound sucks [censored](that's engineering terminology
) . There are lots of "ingredients" that don't show up in testing that the layman sees. In some way, you just have to trust the manufacturer, to an extent. Please don't take this as me saying you are wrong, as you are not.