Modern oil contains a detergent and works hard to remove debris. Wear from moving parts and combustion byproducts are picked up by the oil and flow to the oil pan. Coolant may seep from cylinder-head and intake gaskets and moisture enters from condensation.  These end in the oil pan and circulate back to the oil filter, under pressure from the oil pump.  The oil filter's job is to remove and hold as many contaminants as possible.

Inside the oil filter is a material called the media. Microscopic pores in the media allow oil to pass but trap much of the debris. The size of these pores is critical. If they are too large, debris will pass right through.  If the pores are  too small, the filter may prematurely clog.

When a filter becomes clogged or severely restricted a bypass will open to prevent the engine from running without oil.  This is preferable to running without oil, but unfiltered oil now circulates through the engine.  This unfiltered oil can cause premature wear and damage components.  Substandard oil filters and poorly designed filters offer more risk of such problems.  Even a quality filter may become clogged with extended oil changes.

The oil filter produced by the manufacturer of the vehicle is normally a good choice. Original equipment manufacturer or OEM filters will always meet the warranty requirements for the vehicle.  Some aftermarket oil filters may not have the same quality media as the OEM part. Substandard filters may allow debris to pass through or even become restricted and allow oil to bypass the filter.

We must also properly seal the filter media, within the filter canister.  Failure to seal the media to the canister can allow unfiltered oil to bypass the media.  Cardboard end caps may not be as rigid as metal end caps.

Many OEM filters also have built in anti drain-back valves. This valve helps prevent oil from flowing through the filter, back to the oil pan when the engine is not running. Keeping oil in the upper engine helps with unlubricated start ups. If all oil drains to the pan, it is not immediately available upon startup.  Damage may occur in the engine if oil takes too long to reach its destination.

Overhead cam engines use long chains or belts to drive the camshafts.  These drives are usually kept tight by hydraulic tensioners.  A tensioner has a hydraulic piston and uses engine oil to push against the chain or belt.  Without this pressure there will be a jerk on startup which can break chain guides and cause severe problems. 

Another concern with oil filters, are the threads that hold the filter to the engine. OEM filters have rolled threads that are much stronger than less expensive die-cut thread. Die-cut threads can also produce debris or leave shards of metal in the thread area, which can end in an engine.

Many auto repair shops and even some new car dealerships may use aftermarket oil filters because of their lower price. This may not be in the best interest of the client. A lower price does not mean an overall lowest cost. Saving a few dollars on an aftermarket filter and damaging a six-thousand dollar engine, is not a very good deal. To be sure, always ask if they intend to install an original equipment manufacturer (OEM) oil filter.  If there is any doubt supply the filter and request the shop install it or find another shop.

In recent years some OEM part manufacturers have started second lines. For instance Delco has long been a supplier to General Motors. Delco still produces OEM filters, but also supplies a non-OEM imported line of parts at a lower price. Telling the difference by looking is difficult for the average person.  An unscrupulous operator could pass second-line parts off as OEM. Dealing only with trusted vendors is wise and always asks for an OEM oil filter.