Engine lubrication oil degrades at varying rates depending on the lubricant, engine type and application. Traditional maintenance programs are designed to change oil on predetermined intervals (such as run time/mileage), with more advanced algorithms taking into account load and operating temperature of the engine, or lab analysis. Conservative interval based maintenance programs spend too many resources changing oil and longer intervals may result in engine damage. Lab based oil condition approaches also have significant time lag and other logistical difficulties. Real time engine oil condition analysis offers a balance between oil life and maintenance cost. Real time oil analysis also allows for the implementation of active reliability-centered maintenance. When a fleet manager knows the actual maintenance condition of each vehicle in a fleet, it is possible to accurately prioritize and schedule appropriate maintenance. Real-time oil condition monitoring can help insure that a fleet utilizes the maximum useful life of the lubricants while protecting the performance of the engine Researchers at Symyx Technologies have developed a miniature, solid state oil condition sensor based on a crystal tuning fork. The sensor provides direct measurement of the critical physical properties of viscosity, density, dielectric permittivity, and AC conductance of lubricants [1, 2]. Simultaneous measurements of multiple parameters of lubricating oil in an engine can provide improved sensitivity for detection of changes, which may be the result of degradation or contamination. This paper investigates the response of the Symyx sensor to various diesel engine oil base stocks, additive packages, common oil contamination and real time engine oil monitoring. Results of the Symyx sensor output are compared to conventional lab oil analysis techniques. The suitability of the Symyx sensor as means of determining diesel engine oil condition and predicting remaining useful oil life is discussed.