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Engine oil is very important for engine life and operation. therefore, determining investment period (IP) of oil is useful in minimizing production and harmful wastes cost. Natural gas (NG) maximizes oil drain intervals (ODI) and reduces toxic gas components caused by internal combustion engine (ICE). In addition, it is available in Syria and could be used as vehicle's fuel. This research includes laboratory tests and field trails to determine ODI for Syrian mineral oil API SAE 40/ CF used in gasoline engine when operated on compressed natural gas (CNG). Several used oil samples have been taken from three Bi-fuel vehicles operating at a petroleum field. Each sample was analyzed to monitor change in the physical and chemical properties of the oils, additive depletion and accumulation of contaminants. Used oil analysis (UOA): Kinematic viscosity, total acid number (TAN), total base number (TBN), flash point, antioxidant and anti-wear additives were performed in tribology laboratory. UOA results were used to determine IP for gasoline engine operated on CNG. ODI of (18000 km) were obtained in comparison with original intervals (7500 km) recommended when oil used in gasoline engines. ODI is extended more than two times. These results will affect economy and environment positively. It reduces the cost of engine oil drain, quantities of new oils and used oils wastes by nearly two times.