This study compared several types of well pump lubricating oil for their relative contribution of Assimilable Organic Carbon (AOC) to groundwater. The objective was to determine if the use of an oil-lubricated well pump system could contribute to the growth of bacteria in water distribution systems. A modified Van der Kooij procedure was used to test for the AOC concentration in sterile tap water samples in which one of four types of pump lubricating oil had been added. The resulting concentration was compared to the same water without the addition of an oil. It was determined that lubricating oil can release bacterial nutrients (AOC) into the water and that there is a wide range to which different oils contribute.