Lubricating oil is the combination of base oil and additives which undergo several chemical changes due to oxidation reactions occurring at higher temperatures and pressure. The oxidation reaction produces several oxygenated compounds which lead to degrade the performance of the lubricating oil and adversely affect the engine components, thereby causing frictional and blow-by losses Therefore, the periodic assessment of in-service lubricating oil is necessary to obtain valuable diagnostic information on internal combustion engines (ICE). This paper highlights the results of lubricant degradation analysis carried out on a 4-stroke petrol engine of a newly procured two-wheeler scooter. The experimental studies consider the spectroscopic and rheological analysis of fresh and thermally aged lubricants. Additionally, the effect of thermal degradation on the friction coefficient and wear particle morphology of engine components are also considered in the experimental investigations. The results obtained from oil/wear analysis provide reliable insight to understand the root causes of oil degradation and their effects on friction and wear parameters of engine components operating under normal conditions.