During the operation of lubricated mechanical systems, lubricant properties can change over time depending on the operating conditions, affecting durability and efficiency. This study quantifies the impact of oil degradation on lab-scale gearbox efficiency by testing fresh and field-collected mineral oils. Rheological measurements reveal a decrease in viscosity in degraded oils compared to fresh oils. Thermal and tribological tests using the FZG efficiency test rig were conducted to analyze the different dissipated energy sources (bearing viscous and friction losses, gear tooth friction losses, oil churning losses, etc.). The numerical model used aligns well with experimental results, revealing that field-collected mineral oils differ in efficiency and thermal behavior from fresh oils. As a consequence, testing lubricants only in their fresh condition may not accurately represent real applications. Also, the impact of oil degradation on performance varies with the test conditions, necessitating careful selection to accurately replicate real applications. Finally, it is important to emphasize that this is a preliminary study, in which clear effects of oil degradation were observed for the specific type of mineral oils tested. Therefore, a broader investigation involving a wider range of lubricant types is required, as different formulations may exhibit distinct behaviors in relation to oil degradation.