Originally Posted by Tom NJ
The oxidation value by FTIR does not measure ester base oils per se, it measures carbonyl bonds, C=O. Ester base oils contain C=O bonds, but so do some of the additives. If the oxidation value of an oil is significantly higher than the average conventional oil it usually indicates the presence of some ester base oil content, but how much is difficult to say without knowing which ester is being used.
Ester base oils used in motor oils may contain one, two, three, four, six or more C=O containing ester linkages, so to calibrate the oxidation value against the ester content you need to know which ester(s) are being used and the effect of the additive system. Best you can say is the higher the oxidation value above an average conventional non-ester oil, the higher the ester base oil content is likely to be, but assigning a percentage without composition information and calibration is just a guess.
I agree with Molakule that an ester content of around 5-15% is enough to provide additive solubility and seal conditioning, and more will kick in some cleaning capability and lubricity, but too much can adversely affect seals. Exact numbers depend on which esters and other balancing base oils and additives are used.
Ok, I think I have a good understanding of it now. The oxygen content isn't consistent across all types of ester, and I need to stop overthinking it.
I appreciate the great information in this thread.