The principle of FT-IR (Fourier Transform Infrared) spectroscopy is based on there being different molecules present in the lubricant which, because of their typical chemical structures, absorb infrared light to different degrees with certain wavelengths. Changes to the specimen can be compared to the fresh oil reference spectrum and depicted, calculated and interpreted as typical "peaks" with certain "wave numbers". For the study of filter residue, we use a special variant of infrared spectroscopy. The ATR infrared spectroscopy (attenuated total reflection) is based on a weakened ("attenuated") total reflection. It proved especially effective in the case of checks on opaque materials. The ATR-FTIR provides information about oil mixtures with foreign oil and impurities, such as water, from oil-moistened filter residues. Changes in the case of additive composition (additive sludge) must also be recognised. Through comparison with the deposited spectra of fresh oil, the procedure provides information reliably and quickly whether an unknown oil is a mineral oil, "bio-oil" or synthesis oil.