Engine oil is a valuable source of information on the technical condition of the drive unit. Under the influence of many factors, including operating conditions, time, high temperature, and various types of contamination, the oil gradually degrades, which can result in serious engine damage. The subject of the article focuses on an attempt to answer the questions of how engine failure affects the degradation of engine oil and whether we can use this knowledge to detect potential problems in public transport vehicles at an early stage. The research material consisted of samples of engine oil in the SAE 10 W-40 viscosity class and data on vehicle faults obtained from the service of a public transport company. The oils come from two city buses belonging to the fleet of diesel-powered vehicles, in which critical cooling system failures were diagnosed during engine tests, excluding the vehicle from further use. The conducted studies analyzed the degree of oil degradation, which included determining the changes in kinematic viscosity at 40 °C and 100 °C via a Stabinger SVM 3001 viscometer. The physicochemical parameters of the oil, such as the degree of oxidation, nitration, total acid number (TAN), total base number (TBN), and content of impurities in the form of soot and glycol, were examined via FTIR spectroscopy. The degree of impurities and the general quality of the oil were also determined via a blotter spot test. Additionally, to determine the degree of metal abrasion and changes in the depletion of additives, elemental analysis via the HDXRF method was used. As a result of the tests carried out, potential correlations between the oil condition and the technical condition of the vehicle in real operating conditions were confirmed.