Every lubricant, whether mineral oil-based or synthetic, ages with time. Important factors which accelerate ageing include stress due to high temperatures, air admission, and catalytic processes, as well as the type and amount of any impurities. As oil ages, oxygen attaches to the hydrocarbon chains of the molecule. This produces double carbon-oxygen bonds (carbonyl groups). These carbon-oxygen bonds can be used in the analysis of the oxidised oil to infer its ageing and thus its remaining usability.

Fourier-transform infrared (FT-IR) spectroscopy is used when considering the oxygen bonds or the oxidation of an oil. It provides information about changes to the oil, such as oxidation, as well as any impurities. FT-IR spectroscopy makes use of the fact that, due to their typical binding form, the molecules present in the lubricant absorb infrared light to different degrees at specific wavelengths. For mineral oils, the different degree of absorption resulting from the ageing of the oil is displayed as a “peak“ in the IR spectrum at a certain wave number. According to DIN 51453, a numerical value for the absorption of the IR radiation based on a one-centimetre thick oil layer can be given in A/cm for mineral oils by subtracting the fresh spectrum from the used oil spectrum.

For a mineral oil-based lubricant, even a relatively small „peak“ which increases continuously in the used oil spectrum can be interpreted as a clear sign of increasing oil oxidation. However, when determining the oil oxidation of lubricants with ester-containing base oils and/or additives dissolved in esters, FT-IR spectroscopy reaches its limits. Carbonyl bonds are inherent to ester oils, which absorb infrared light at a wave number of approximately 1,740 cm-1, and thus in the same range as the carbonyl bonds which are created as a result of the oxidation of the oil. In the infrared spectrum of a fresh ester-containing lubricant, an oversized „peak“ in the wave number range of approximately 1,740 cm-1 is formed by its carbonyl compounds alone. In contrast, if the used oil is examined, a change in this dominant „peak“ due to possible oil oxidation can no longer be calculated. Although it is no longer meaningful to determine the oxidation value for lubricants with ester-containing base oils and/or additives dissolved in esters by means of FT-IR spectroscopy, its determination according to DIN 51453 is still specified in the fuel specifications of several OEMs. Up to now, only the engine manufacturer MWM has taken these facts into account. The MWM specification for gas-engine lubricating oils does, indeed, indicate a limit value of 20 A/cm for oxidation according to DIN 51453. But there is a note: „The determination of the oxidation does not apply to lubricants containing synthetic esters.“