Motor oil carries hazardous engine deposits away from the engine.

An oil filter removes these impurities from the lubricant, so that it can continue to protect the engine parts. It is the main line of defense against abrasion and engine deposits, and should be renewed in conjunction with an oil change before it becomes clogged. Full flow filtration systems in today's cars pump the lubricant through to the filter before it reaches the engine parts. Like the oil itself, the filters vary, and each different kind has different effects on each engine. 

The two basic types of oil filtration used in cars are spin-on and cartridge filters, although most cars today are made to fit cartridge filters. However, this can vary by manufacturer: though many have switched back to using the cartridges, the spin-on variety was the standard for years. The spin-on filters are fashioned inside a metal can which unscrews from a mount on the engine. This makes installation and replacement during an oil change much easier and cleaner than the cartridge filters. The cartridge cost less because it uses less materials, but it tends to drip excess oil in the process of replacing it with a new one.

The materials used to make the filter can have a difference in the effectiveness of the filtration. Single-pass efficiency, multi-pass efficiency, and micron rating are the three main criteria for determining how well an oil filter will keep the lubricant clean. The filter's ability to remove contaminants on a single pass through the filter helps determine the single-pass efficiency.

Subsequent passes through the filter determine the multi-efficiency rating. An acceptable filter will eliminate at least 98 percent of the impurities on the fast pass. However, the multi-pass rating is a better indicator of the filter's efficiency because contaminated oil will continue to run through the engine. An ideal multi-pass rating is at 96 percent of the impurities that pass through the filter after the first time. The micron rating is a measurement of what size particles are allowed to pass through. An ideal micron rating is 10, this means that large particles cannot pass through the filter and damage the engine; larger particles tend to cause more damage. The filter's ratings can vary depending on the type of materials used to make the filter and its thickness.

Choosing the right filter is just as important as choosing the right oil when getting an oil change. It should be changed regularly along with the oil because it loses its effectiveness if it is already full of particles and impurities from the used oil. Refer to the owner's manual for your vehicle to determine the best filtration system for the type of engine and conditions.