The Oil Filter Explained: What It Does and When It Changes

The oil filter is the one part of an oil change that nobody sees, and most owners could not say whether theirs is a metal can or a paper cartridge. It is doing a lot of work. Every quart of oil passes through it many times an hour, and it holds whatever the engine sheds between changes. This guide explains how it works, the two common designs, the bypass valve that saves the engine when the filter clogs, and why a new filter goes on with every oil change at our service centers.

At a glance

• The filter catches wear metal, carbon, dirt and sludge before they reach the bearings.
• It is replaced with every oil change; a used filter puts old contaminants into new oil.
• Spin-on filters are a sealed can; cartridge filters are a paper element inside a housing on the engine.
• A bypass valve lets unfiltered oil through when the filter is clogged or the oil is cold and thick, which is why a loaded filter is worse than no protection at all.

What the filter actually catches

Engine oil picks up three kinds of contamination. Wear metal comes off the bearings, rings and cam lobes in tiny amounts every mile. Carbon and soot come from combustion and blow past the rings, especially in direct-injection and diesel engines. Dirt comes in through the air intake and the crankcase breather, and in a high-desert climate with about 7 inches of rain a year at the Reno airport (NOAA 1991-2020) there is plenty of it. The oil's dispersants keep these particles suspended so they do not settle as sludge, and the filter's pleated media pulls them out as the oil circulates. Over an interval the media fills up, which is the whole reason the filter changes with the oil.

Spin-on versus cartridge

A spin-on filter is a sealed steel can with the media, an anti-drainback valve and a bypass valve inside. It threads onto the engine, the whole can is replaced, and the old one is drained and recycled. Most trucks and many older cars use them. A cartridge filter is a bare pleated element that drops into a housing built into the engine, usually on top where it is easy to reach, with a cap and an o-ring that are replaced with the element. Many European makes and a growing number of Japanese and domestic engines use cartridges because they generate less waste. Both designs work the same way and the right one is whichever your engine was built for; the part number comes from the manufacturer's specification, and we match it.

The bypass valve and the anti-drainback valve

Two small valves inside the filter matter more than the media on a cold Minden morning. The anti-drainback valve keeps oil in the filter and the galleries when the engine is off, so the top of the engine gets oil pressure seconds sooner on start-up instead of running dry while the pump refills the filter. The bypass valve opens when the pressure drop across the media gets too high, either because the oil is thick and cold or because the media is clogged. Oil then flows around the filter unfiltered, which protects the bearings from starvation but sends every particle through the engine. A filter that went two intervals without a change is likely to spend part of every cold start in bypass. That is the engineering reason for a fresh filter each time, not a sales reason.

Why the filter is changed with the oil

A used filter holds everything the last fill collected. Put fresh oil through it and the new oil inherits a loaded filter that is closer to bypass and a quart or so of old oil that never drained. The filter is a small fraction of the cost of the change, and the manufacturer's schedule nearly always lists oil and filter together for that reason. Some manuals for cartridge engines list the filter at every other change; we follow the manual when it says so and tell you which case applies. The oil change page lists what the full service includes.

The filter in the bay

Every full-service oil change at our four service centers includes a new oil filter matched to the manufacturer's specification for your engine, whether that is a spin-on can or a cartridge with its o-ring and cap seal. The technician checks that the old gasket came off with the old filter, since a doubled gasket is the classic cause of a sudden oil leak after a change, lubricates the new gasket, and tightens it to spec by hand rather than with a wrench. After the fill we start the engine, confirm the pressure light goes out, and look for seepage at the filter and drain plug before the car comes down.

Longer intervals make the filter matter more, not less. If your manual allows 7,500 or 10,000 miles on synthetic, the filter is carrying the engine's debris for all of those miles, and a quality filter rated for the interval is part of what makes the long interval safe. Drive into Mae Anne Avenue, Longley Lane, College Parkway or US 395 in Gardnerville and the right filter goes on with the right oil, no appointment needed.

Signs of a filter problem

• Fresh oil spots on the driveway right after a change: a doubled or pinched gasket.
• The oil pressure light flickering at cold start: an anti-drainback valve that is not holding, or a clogged filter.
• Oil that turns black within a few hundred miles: the filter was not changed or the engine is sooting heavily.
• A ticking top end on cold mornings that goes quiet after a minute: slow oil delivery, sometimes a tired filter.