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

Oil filters are used in many different types of hydraulic machinery. A chief use of the oil filter is in internal-combustion engines in on- and off-road motor vehicles, light aircraft, and various naval vessels. Gas turbine engines, such as those on jet aircraft, also require the use of oil filters.

Mechanical
Mechanical designs employ an element made of bulk material (such as cotton waste) or pleated Filter paper to entrap and sequester suspended contaminants. As material builds up on (or in) the filtration medium, oil flow is progressively restricted.

Cartridge and spin-on
A self-contained housing and element assembly which was to be unscrewed from its mount, discarded, and replaced with a new one. This made filter changes more convenient and potentially less messy, and quickly came to be the dominant type of oil filter installed by the world's automakers.

Magnetic
Magnetic filters use a permanent magnet or an electromagnet to capture ferromagnetic particles. An advantage of magnetic filtration is that maintaining the filter simply requires cleaning the particles from the surface of the magnet.

Sedimentation
A sedimentation or gravity bed filter allows contaminants heavier than oil to settle to the bottom of a container under the influence of gravity.

Centrifugal
A centrifugal oil cleaner is a rotary sedimentation device using centrifugal force rather than gravity to separate contaminants from the oil, in the same manner as any other centrifuge.

High efficiency (HE)
High efficiency oil filters are a type of bypass filter that are claimed to allow extended oil drain intervals. HE oil filters typically have pore sizes of 3 micrometres, which studies have shown reduce engine wear.