According to the quality of gasoline, it is best to replace the car when it runs about 30 thousand kilometers, but it should be replaced with genuine products. Failure to replace it for a long time will damage the oil pump and injector. Oil-water separator is a kind of machinery, which is usually used for restaurant sewage treatment. Due to the requirements of environmental protection, the sewage discharged to the ship machinery places in rivers, lakes and seas must be treated and an oil-water separator is needed. Generally, the oil-water separator should be replaced every 8000- 10000 km for diesel vehicles. The functions of the oil-water separator are as follows: 1. Oil-water separator is a separator that removes impurities and moisture according to the density difference between water and fuel, and there are separation elements such as diffusion cone and filter screen inside; 2. The oil-water separator has other functions, such as preheating fuel to prevent wax deposition and filtering impurities. Moreover, oil-water separation has a certain effect on improving engine oil combustion; 3. It is a kind of fuel filter, and its main function is to remove water from diesel oil, thus reducing the faults of fuel injectors and prolonging the service life of engines. It is better to change it from 8000 to 10000 km. If there is water and impurities in diesel oil, the parts of fuel system will be corroded and rusted by the water in diesel oil. Removing water from diesel oil is very important for trouble-free operation and prolonging service life of fuel system. The oil-water separator plays this role.