Q: why oil filter be replaced ?

This is a very good question that I wish more people would ask. The reason why you change the filter at the same time you change the oil is due to the fact that the filter has a "lifespan" just like the oil does. The oil filter is constantly removing carbon and other deposits from the combustion process from the oil. When the cylinder inside your engine ignites the fuel from the gas tank, it leaves a dark residue of carbon. As the piston slides up and down the cylinder, oil lubricates these components and removes the carbon from the combustion chamber so it doesn’t scratch or cause damage. This is why oil turns black as it is used more frequently. The filter’s lifespan is about the same as the oil (recommended every 5,000 miles).