In this paper, estimation of the chemical composition of used oils collected from several European locations was performed on the basis of a comparative analysis of chemical composition of commercially available fresh and used motor oils. Although the motor oil undergoes a range of chemical and physical transformations during routine engine operations, information about the structure of hydrocarbons in the fresh oil allows for an estimation of the approximate ratio of different types of hydrocarbons in the same oil after its use. As an example, a particular type of fresh oil was used in the engine and then reanalyzed by the same analytical techniques. Gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis (CHNS/O analysis) were used to characterize the chemical composition of the oil samples. A comparison of the obtained results showed correlations between chemical properties of the fresh oil and the collected used oil. Both oil FTIR spectra exhibited the bands that are related to the presence of carbonyl groups and amine-containing compounds, respectively. Opposite from the fresh oil, phenols were not found in the used oil. According to the obtained H-1 NMR spectra, the paraffinic hydrocarbons of the fresh oil are more linear and have longer chains than those in the used oil.