An oil filter is a filter designed to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil.

The strainer is a type of filter used mostly on low pressure or suction side and is used to remove large contamination particles from the system. It is because the mesh screen openings in the strainer are bigger in size and are similar to those of coarse filter.

If the oil filter is not changed periodically, the filter can become severely clogged, reducing the volume of oil passing through the filter and into your engine. Without this oil, your engine can suffer severe damage. However, the oil would be dirty, unfiltered oil that has bypassed the oil filter.