###### Contents  
* 1 Introduction
* 2 Combustion engine users frequently replace engine oil that is still usable or keep using engine oil that is contaminated and no longer suitable for use. Engine oil is exposed to various contaminants and oxidation that affect lubricant efficiency, and thus the oil has a limited life. Additives for engine oils reduce the effect of contaminants, oxidation, and prevent corrosion [1]. For practical reasons, designers of internal combustion engines recommend changing engine oils after a specified period of operation, even though they know working conditions and failures of identical engines systems are not the same. For instance, two identical engines may even be operating under the same load and external conditions, but the second engine has a gasket failure allowing coolant (containing glycol) to leak into the engine and mix with the engine oil. While the life of the first engine will probably not be reduced by waiting for the designer-specified oil change, the second engine may require a very expensive complete renewing process due to poor lubrication and bearing failure prior to the specified schedule of oil exchange. Engine oil is contaminated by glycol due to leakage from an engine's cooling system into the lubricating system, typically caused by a damaged gasket or seal [2]. Glycol mixes with engine oil and reduces lubricity [3] due to glycol oxidation [4] causing sludge in the engine oil and varnish deposits on piston rings and valves [5], plugged lubricant screens [4], and possible engine seizure [6]. A general, acceptable limit of glycol in engine oil has not been determined so far. [PERSON] [7] reported that 150 ppm of glycol did not damage engine oil. [PERSON] [8] found that 500 ppm of glycol was not detrimental to engine oil. However, any glycol contamination of engine oil is a concern since it generally originates from a leak and contamination generally increases. Contamination and aging of engine oil can be determined by using established methods, such as ASTM International (ASTM) standards and methods. Used engine oil samples may be collected and sent to one of several labs that will analyze the oil using ASTM or modified-ASTM procedures and provide a report summarizing the contaminants. While some industrial users make regular use of such services, most vehicle owners do not, as it would be too cumbersome and time-consuming. Engine users worldwide would benefit from in-situ technology for real-time engine oil analysis to alert users of contaminants or aged engine oil. Since glycol has low molecular weight and volatility but a high polarity, it is difficult to detect and quantify in engine oil. Hyperspectral [9], electromechanical [10], infrared spectroscopy [11], and resonating sensors [12] were assessed for potential in in-situ sensor systems. These sensors are designed to monitor oil conditions in terms of viscosity [13], acidity [14], antioxidants [15], and water or freezing contaminants [16]. However, none of these sensors could identify all the contaminants, and multiple sensors were recommended to monitor oil conditions [17]. While many recent automobiles have some indicator of when to change engine oil, the sensor systems are primarily based on mileage rather than the direct sensing of oil contaminants. New technologies should continue to be designed and evaluated for potential future in-situ engine oil monitoring. Scientists continue to develop sensors, including spectral analysis and, most notably, terahertz spectroscopy, which has been applied to many applications [18; 19].