A Mazda oil filter is a particle remover that must be in proper functioning order for the longevity of your engine. Because of this, the filter element inside of this oil component will fill up and need replacing on a regular basis. The particulates that the oil filter removes from the engine oil are being produced by the Mazda engine itself. This is the solid components from the ignition of the fuel in the combustion chamber. They are very fine particulates that are blown past the piston rings down into the crankcase of the motor in your Mazda. The buildup of these particulates in the engine oil is only reduced by the oil passing through the oil filter element. The recommendation of changing the oil and oil filter at 3,000 to 5,000 miles was computed because this is the time frame in which the filter element inside of the Mazda oil filter will begin to be clogged do to this build up of particulates. For a short period after that milestone, the oil filter will still function, but as time goes on, its capacity to remove the particulates will be reduced. Fully synthetic oil will lengthen this interval to 7,000 to 10,000 miles. There is a level of blockage the filter has where the reduction in the oil flowing through it is not sufficient to supply the bearings in the Mazda engine with the appropriate amount of lubrication. When this point is reached, the check valve will close off the oil filter. This makes it possible for the oil pressure to be maintained so the bearings and other moving parts in the engine can be lubricated, but the oil is contaminated with particulates. The oil is not worn out or bad; just filled with particulates that will damage the bearings if they are exposed to them in a large enough concentration for an extended period of time.