Laboratory Diagnostics of Engine Oils as a Tool for Identifying Mechanical Faults and Supporting Sustainable Vehicle Maintenance

This study presents a comparative laboratory analysis of seven engine oil samples collected from passenger vehicles exhibiting significant viscosity deviations after operation. The aim was to demonstrate the diagnostic value of physicochemical testing in identifying mechanical or operational malfunctions that may not be detectable through routine vehicle servicing. Each oil sample was characterized by measurements of kinematic viscosity at 40 °C and 100 °C and Fourier-transform infrared (FTIR) spectra. The combination of these methods enabled the assessment of oxidation, fuel dilution, additive depletion, and contamination levels. The findings reveal consistent patterns linking abnormal viscosity reduction or increase with distinct spectral changes, particularly in the regions associated with oxidation (1710–1740 cm −1 ), sulfation (1150–1250 cm −1 ), and nitration (1600–1650 cm −1 ). The study highlights that in cases of pronounced physicochemical anomalies, the use of alternative oil brands or viscosities cannot compensate for underlying mechanical issues. Laboratory diagnostics, combining viscosity testing and FTIR spectroscopy, provide an effective approach to pinpointing such faults, thus supporting preventive maintenance and informed decision-making in engine servicing. The proposed approach contributes to sustainability by enabling condition-based maintenance, reducing unnecessary oil replacement, and minimizing environmental impact associated with lubricant waste.