Characteristics of Quality Oil Filters

The characteristics of a high quality oil filter are highly desirable, but not always externally visible. Most auto parts retailers offer a wide variety of oil filters, and aside from being different colors, they all look almost the same. A visual inspection can still reveal some important points, such as case construction, end cap design, and o-ring seal mounting. However, many of the most important factors that determine filter quality are internal, and their performance is only determined through laboratory testing. Still, some research will help you select the best quality filter for your application at the best price.

A good filter has a strong steel case to withstand the high oil pressure (60-80psi when cold), an anti-drainback valve that effectively prevents flow reversal while creating minimal restriction, a pressure relief valve with a positive seal, and a strong element and cap that can withstand the pressure and flow of oil without becoming damaged. The element media has to be able to strike an acceptable balance between dirt containment and flow restriction.

It should be noted that while most oil filters contain media in a steel canister that is threaded on to the engine, some automakers use a permanent canister that uses a disposable filter media cartridge that can be removed and replaced.

The typical spin-on filter has an end cap with a threaded center hole, surrounded by a group of smaller holes. Oil enters through the surrounding holes, passes through the filter media, and exits through the threaded center. The outer holes must be of sufficient quantity and size to allow unrestricted oil flow into the canister. The center return hole should be well formed and free of burrs or damaged threads. The end cap itself must be sturdy enough to resist flexing, which can create leakage when the unit is tightened. Most quality oil filters feature plated end caps to prevent rust before they are installed.

The surface of the end cap is sealed to the engine block using an o-ring gasket to prevent leakage. The o-ring should be solidly attached to the end cap. The quality of this o-ring can vary, with cheap filters using lower quality material, which can either soften or become brittle with age. Either condition can lead to seal failure, creating a leak at a critical point in the oiling system. If such a leak is severe enough, it can lead to engine damage or destruction due to loss of oil pressure and supply. Some high quality oil filters will use an improved seal material, such as Viton, which is more resistant to heat and oil exposure.

Most filters have an anti-drainback valve to prevent oil from draining backwards into the oil pan. Cheap filters, however, may have valves that fail to seal properly. When this happens, oil drains out of the filter when the engine isn’t running. Many engines will exhibit excess valvetrain noise at start-up until the filter fills up again and oil begins to circulate through the engine. This period of oil starvation contributes to unnecessary wear.

Most filters also have a pressure relief or bypass valve that will allow oil to flow past the filter element in the event that it becomes too clogged for the oil to flow through it. This prevents oil starvation, which will severely damage the engine. Similarly, a bypass valve is necessary to prevent bursting the element, possibly causing pieces of it and the debris it filtered to enter the engine, again causing serious damage. Also, when the oil is cold it may become very thick, and will tend to bypass the filter through the pressure relief valve because it cannot pass through the element until it warms up and thins out somewhat. If it did not do this, the filter element media would again be in danger of bursting.

The filter media is the part we usually pay the most attention to. The filter media is usually a cellulosic (paper) or a synthetic material, although different media, such as steel wool or cotton, have been used over the years. Paper media is used on inexpensive filters and works reasonably well. The paper is generally treated with some type of resin or coating to improve the performance of the media and help it withstand the rigors of the environment it operates in. The fibers are able to block particles down to a certain average size, while allowing the oil to pass through. On average, paper elements can trap about 97 percent of the 40 micron particles, 93 percent at 30 microns, 60 percent at 20 microns, and 40 percent at 10 microns.

Some manufacturers add other media, such as cotton, to the cellulose to improve its performance. High-end filters often use synthetic fiber media that has smaller passages to trap smaller particles, but can pass more fluid through it because it has a greater total number of passages. There is also media that is a hybrid of these two.

Then, there are units known as depth filters; as the oil passes deeper into the element, the smaller the passages get. This way, large particles are trapped toward the front surface of the element while small particles are stopped deeper in the element. This allows the filter to hold more debris before it becomes full.

The characteristics of a quality oil filter are not always obvious, the benefits to using a high quality filter are. And while it may take some research to determine which filters are built well, the extended engine life and reduced operating cost will pay you for your effort.