Lubricant degradation under in-service conditions can be often traced to oxidation. Environmental effects under in-service conditions are often recognized to affect the oxidation pathways of lubricants. In this study, environmental factors such as metal catalysts, fuel components, and oxides of nitrogen are studied systematically in a simple oxidation test. The test consists of bubbling air into the oil sample at 175°C. Periodic samples were withdrawn and analyzed. Results show that the environmental factors not only have significant influence on the rate of oxidation, but also on the oxidation pathway of how lubricants degrade. The acid number increase was found to be an important parameter in monitoring the lubricant degradation mechanisms.