This study investigates fuel-induced oil dilution in hybrid powertrains using a combined assessment of kinematic viscosity and FTIR differential spectroscopy. Ten oil samples collected from hybrid vehicles operating under diverse real-world driving patterns were examined to determine how hybrid-specific operating conditions—such as frequent cold starts, extended start–stop phases and short, thermally unstable trips—influence lubricant ageing and, consequently, the energy efficiency of the combustion subsystem. In eight of the ten cases, a clear reduction in kinematic viscosity was observed, indicating the presence of volatile fuel fractions and confirming that fuel dilution is a dominant mechanism shaping the early stages of oil degradation in hybrid engines. FTIR analysis consistently revealed spectral shifts related to oxidation, nitration, sulfonation and additive depletion, together with hydrocarbon enrichment characteristic of fuel contamination. The co-occurrence of viscosity loss and FTIR band evolution demonstrates a strong and reproducible relationship between mechanical thinning of the lubricant and chemically driven transformation pathways, both of which can negatively affect frictional losses and energetic performance. Paper-based blot testing was used only as a supplementary qualitative tool and provided visual confirmation for samples exhibiting the strongest fuel-related FTIR signatures and viscosity changes. Although not mechanistically specific, the method reinforced the laboratory findings in cases of pronounced degradation. Overall, the results highlight the diagnostic value of combining viscosity data with FTIR spectral analysis to characterise fuel dilution and associated ageing mechanisms in hybrid combustion systems. This study contributes to a more comprehensive understanding of lubricant deterioration under real hybrid driving conditions and supports the development of practical monitoring strategies aimed at safeguarding both engine durability and the energy efficiency of hybrid powertrains.