(From recent article published on USGS web site)
Methyl tert-butyl ether (MTBE) detections in ground water may not always indicate contamination from leaking gasoline storage tanks, which have a significant potential for large-scale contamination that exceeds regulatory standards. USGS scientists have discovered that used motor oil can contain a significant amount of MTBE. The scientists tested new and used motor oil for MTBE and found that the used oil contained on the order of 100 milligrams of MTBE per liter. MTBE was absent in the new oil tested. Apparently the oil in motor engines absorbs MTBE and other compounds (like BTEX) in gasoline as the oil circulates through the engine. The United States generates 1.3 billion gallons of used motor oil each year-approximately 13 percent of which is disposed of improperly (Motor Oil Facts). Improperly disposed of motor oil therefore could potentially be a widespread source of MTBE in ground water. Previously, USGS scientists identified the atmosphere as a potential source of low-level MTBE concentrations in shallow ground water. Environmental professionals and water resource managers can use this information to make informed decisions about the potential sources of relatively low levels of MTBE in shallow ground water.