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What Every Motorist Should Know About Oil Filters

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replace oil filter

The oil filter is the engine's main line of defense against abrasion and the premature wear. The oil filter removes solid contaminants such as dirt, carbon and metal particles from the oil before they can damage bearing, journal and cylinder wall surfaces in the engine. The more dirt and other contaminants the filter can trap and hold, the better.

In today's engines, all the oil that is picked up by the oil pump is routed through the oil filter before it goes to the crankshaft bearings, cam bearings and valvetrain. This is called "full-flow" filtration. It is an efficient way of removing contaminants and it assures only filtered oil is supplied to the engine. In time, though, accumulated dirt and debris trapped by the oil filter begin to obstruct the flow of oil.

engine oil system
The oil filter cleans the oil before it is recirculated back through the engine.

The oil filter should be changed before it becomes clogged. Since there is no way to tell how dirty the filter actually is, the filter should be changed according to the maintenance schedule in your vehicle owners manual, or when the oil is changed.

Though some vehicle owner's manuals say oil filter replacement at every other oil change is acceptable, most professional technicians always change the oil filter at every oil change. Why contaminate the clean oil with up to a quart of dirty oil? And why risk expensive engine damage if the old filter is on the verge of clogging up? A new oil filter is cheap insurance against premature engine wear.


If you wait too long to change the oil and oil filter, there is a danger that the oil filter might become plugged. To prevent a plugged oil filter from starving the engine for lubrication, oil filters have a built-in safety device called a "bypass valve." When the differential pressure across the oil filter element exceeds a predetermined value (which varies depending on the engine application), the bypass valve opens so oil can continue to flow to the engine. But when the bypass valve is open, no filtration occurs.

The bypass valve also opens when a cold engine is first started. Cold oil can be fairly thick and may not pass through the filter element very easily. So the bypass valve opens and allows the oil to go around the filter until the oil warms up and flows more easily. During this time, any contaminants that are in the crankcase may be sucked up through the oil pump and bypass the filter, causing increased engine wear and possibly engine damage. Once the oil gets warm and the bypass valve closes, oil flows through the filter and normal filtration resumes.


Most oil filters look alike on the outside. One might conclude, therefore, that all oil filters are the same in their filtering capacity and ability to protect the engine from wear particles. But all oil filters are NOT all the same. There can be significant differences that affect both filtration efficiency and longevity.

Some oil filters may contain up to 50% more square inches of filter media than a cheaply made oil filter for the same engine application! Which oil filter would you rather have on your engine? One with increased filtering capacity or one that may not go the distance?

The ability to trap and hold contaminants is another difference between filters. Oil filters with resin impregnated pleated paper (cellulose) elements are good at removing particles down to about 25 to 35 microns in size. One micron equals a millionth of a meter or 0.000039 in. By comparison, a human hair is about 60 microns in diameter! Yet according to some filter manufacturers, particles as small as 10 to 20 microns can also cause damage over time. To trap these smaller particles, many oil filters now use a synthetic filter media (like synthetic glass fiber), or a media that blends synthetic glass or rayon "microfibers" with cellulose fibers to increase the filter's ability to trap small particles.

Contaminants that are too large to pass through the microscopic pores between the fibers become trapped in the oil filter. As the contaminants build up, they actually increase the filtering efficiency of the media. But after a time the accumulated debris also begins to restrict the flow of oil through the filter. Consequently, an oil filter should have plenty of dirt holding capacity, too. Some "long life" oil filters can hold up to 40% more dirt than a standard oil filter.

Watch Out for Debris Inside New Oil Filters! This should NEVER happen, but sometimes you'll find loose debris such as metal particles or bits of filter media loose inside a brand new filter. Turn the filter upside down and shake it hard a few times or bang it on a table to dislodge any junk that might be inside the filter BEFORE you install it on your vehicle. Debris inside a filter will pass through into the engine's oil system and may jam the oil pump pressure relief valve open causing a loss of oil pressure. Debris can also plug oil passages to the crankshaft or cam bearings, lifters or pushrods, resulting in expensive engine damage.

Watch Out for Fake Filters!

Watch out for fake or counterfeit oil filters that are knock-offs of name brand products. These are often sold online at low "discount" prices, but they are no bargain! The filter may be packaged in a box with a name brand on it (which is counterfeit) or painted and labeled to look like the genuine product (which it is not). The filter media inside these fake filters is typically a low quality pleated paper cellulose (not synthetic fiber or synthetic media) that cannot provide the longevity or filtering efficiency of an OEM filter or quality aftermarket name brand replacement filter. Some of these knock-off filters may also be missing a required anti-drainback valve, or the pressure relief valve may open at too low of a pressure for the application. Some filters also use thinner gauge steel for the canister housing, which may leak or blow out causing a catastrophic loss of oil pressure!.


Oil filters are a high volume mass produced product, so it's not unusual that an occasional "bad" filter comes off the assembly line. It can happen with ANY brand of filter, but there is more of a risk with cheaply made "economy" filters. By bad, we mean the filter may not be assembled correctly internally (the ends of the filter media are not completely crimped or sealed at both ends inside in the can), or the oil pressure bypass valve is defective and is leaking, or there is some residual loose filter media inside the can, or the can itself leaks.If the filter fails to route all of the oil through the filter media because of internal leakage, it won't do a very good job keeping the oil clean. It may allow debris to pass through the filter that can damage the bearings.

If the internal pressure relief valve is not holding pressure because of damage or misalignment, unfiltered oil may bypass the filter element. Dirt or debris in the oil can cause bearing wear and damage.

If there is loose filter material inside the can, it will be flushed out and into the engine's oil system. If the material ends up in the crankcase, it will be draw back into the oil pump. The material is soft so it won't damage the pump gears, but it may jam open the pump's pressure relief valve causing a drastic drop in oil pressure.

Finally, if a spin-on filter canister leaks because the end cap does not seal tightly against the can, loss of oil will create a mess and eventually a loss of oil pressure (and engine damage) if the oil level gets too low.

oil filter exploded view


Follow the filter supplier application catalog to find the right filter for your engine. Oil filters that appear to be the same may in fact have a different thread size or internal valving. The hole in the bottom of a spin-on filter must be the same diameter as the original and have the same type of threads (SAE or metric). If the hole size or threads are different, the oil filter may not fit properly, leak or damage the mounting. The gasket must also be in the same location to seal against the engine. If the diameter of the gasket is too large or too small, it may leak.

Replacement oil filters should have the same internal valving as the original. Many overhead cam engines require an "anti-drainback" valve inside the filter to prevent oil from draining out of the filter when the engine is shut off. This allows oil pressure to reach critical engine parts more quickly when the engine is restarted. Oil filters that are mounted sideways on the engine typically require an anti-drainback valve.

spin-on and cartridge style oil filters

Oil Filter Manufacturers:

Oil filters are made by a number of different manufacturers. Filters may be sold under the manufacturer's own brand name. These manufacturers may also produce filters under different brand names or private labels for retailers and other aftermarket and original equipment parts suppliers.

AC Delco Oil Filters

Bosch Oil Filters

Fram Oil Filters

Hastings Oil Filters

K&N Oil Filters

MAHLE Oil Filters

Purolator Oil Filters

WIX Oil Filters


Until recently, most passenger car and light truck engines used a spin-on style oil filter with a filter element inside a sealed metal can. Fram was the first to add a non-slip coating on the outside of their spin-on filters, which makes makes removal and installation much, much easier, especially if your hands are greasy. Some other filter manufacturers have copied this feature and have added a non-slip coating to their spin-on filters.

One of the drawbacks of spin-on oil filters is that they can sometimes be difficult to remove if the filter was over-tightened when it was installed. Some type of filter wrench is almost always needed to loosen them, and even then the filter may refuse to budge. As a last resort, a large screwdriver can be driven into the side of the thin metal filter housing and used like a handle to loosen the filter.

oil filter
Spin-on oil filters are usually located under the engine.

Another disadvantage of spin-on filters is the risk of the filter not being tightened properly when it is installed. Most filter manufacturers say a spin-on filter should be turned approximately three-quarters of a turn as soon as the rubber gasket on the end of the filter makes contact with the filter seat. The rubber gasket should be lightly lubed with motor oil before it is installed, and you should make sure the filter mount on the engine is clean and smooth (no deep scratches or pits). Also, make sure gasket from the old filter is not stuck to the mount on the engine (it sometimes separates from the filter when the filter is removed). You want the filter tight, but not too tight as this may damage the seal and cause a leak.

If a spin-on oil filter is not tight enough when it is installed, it may leak, or worse yet, come loose causing the engine to lose all oil pressure (which can destroy the engine!).


Most newer engines have replaced spin-on oil filters with a cartridge style oil filter that does not have a metal can around the filter. The cartridge design actually dates back to the 1950s. But the original cartridge style filters were messy to change. That's why spin-on filters cam about. Spin-on filters were faster and easier to change. Engineers decided to return to the cartridge style filter because it was cheaper to produce and reduced the amount of waste going to landfills. Spin-on oil filters could be recycled by cutting the can off the filter to salvage the steel can, but few people were actually going to the effort to do it. With a paper or fiber cartridge filter, there is no metal around the filter so the old filter can be burned or crushed for disposal. Getting rid of the can also reduced the manufacturing cost of the filter.

The new cartridge style oil filters are typically located on top of the engine, rather than underneath, making them easier to change on many (but not all) applications. The filter element is located inside a plastic or metal housing with a screw-on cap. The cap usually has a large hex nut on top.

To change a cartridge style oil filter, you simply unscrew the cap, pull out the old cartridge, drop in the new filter, replace the o-ring on the cap and screw it back on. Make sure this o-ring is properly located in the cap otherwise the oil filter housing may leak when the engine is started. Also, do not over-tighten a plastic cap as doing so may damage the cap or seal.

cartridge oil filter housing

cartridge oil filter housing


To change the oil filter on a newer vehicle that used a cartridge style filter, all you need is a wrench or six-point socket to fit the large hex nut on top of the filter housing. Do not use pliers or Vice Grips to loosen the cap because you risk damaging or breaking the plastic cap. If you are using a socket wrench to loosen the cap (recommended), use a six-point socket rather than a 12-point socket to reduce the risk of the socket slipping and damaging the hex nut on the cap. And when you tighten the cap, just make sure it is fully seated and snug. Do not over-tighten as this may damage or break the cap.

Lisle offset oil filter wrench
Lisle offset hex wrench for removing the cap on a hard-to-reach oil filter housing

To change a spin-on oil filter, you will need a special filter tool or wrench that fits the filter on your engine. If you own more than one vehicle, you will probably need an assortment of different oil filter wrenches to fit different sized oil filters. Filter diameters range in size, so you may need a small, medium and/or large size wrench to grip a range of filter sizes. Some filter wrenches have an adjustable rubber strap that can fit almost any sized filter. The rubber band also tends to grip better and slip less than a metal strap style filter wrench.

Some spin-on filters that are mounted under an engine can be very difficult to reach with a strap-style oil filter wrench because of their location. For hard-to-reach applications, a cup-shaped wrench that fits over the end of the filter usually works well. The wrench grips the flutes on the end of the filter to spin it loose.

oil filter wrenches
Assorted oil filter wrenches

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