From our insanely detailed guide:

Oil filters

Oil is pumped through the filter before circulating around the engine. The filter removes small particles of dirt and tiny fragments of metal which are suspended in the oil. Unless removed, this particles will dramatically increase wear between parts.

In the illustration, the oil filter is attached directly to the main oil gallery. Filters are generally attached towards the bottom of the engine so that oil can flow through them first on its journey up through the engine.

Oil filter types

The first replaceable oil filters were cartridge types, where a replaceable paper filter element sits in a permanent metal housing. In the 50’s, spin-on type oil filters were developed which are the most common type today. In the spin-on type, the metal housing and filter element are replaced together - which is faster and makes less mess than replacing the filter element itself.

These spin-on, or screw-fit, oil filters are somewhat wasteful of materials as the housing is perfectly serviceable when changed but it thrown away regardless. There has thus been a trend back towards cartridge elements since the 90s - the illustrated filter is from a Volvo.

Oil filters are almost universally of a screw fit type - and should be replaced at regular service intervals specified by the manufacturer. Inside the filter housing is a paper or cotton filter element.

Anti-drain valve

When the engine stops running, the oil in the filter will attempt to drain downwards under gravity - the position of the filter on the engine will dictate how much this happens. If the oil drained out of the engine every time it stopped, then parts would be running dry for a short time before the was pumped back around. To prevent this, oil filters may include a anti-drainback valve to prevent this draining from happening and allow an immediate oil pressure on engine startup.

Bypass valve

All the engine oil passes through the oil filter before circulating around the engine. If the filter were to become blocked then no oil whatsoever would reach the engine. To prevent this, as a last chance effort to save the engine, there will be a bypass valve which will open under high pressure to bypass the filter and allow unfiltered oil to lubricate the engine.

Continue reading: Engine lubrication