Oil Filters

As vehicle engines have continued to evolve, so has the requirement for more advanced filters due to increased engine power, bearing loads, engine rpm and operating temperatures, among other factors. However, the main purpose and function of an engine oil filter has remained unchanged: to keep the lubricant clean by removing abrasive particles.

The media in the oil filter is the physical mechanism used for contamination control.

It could be said that the oil is the blood of an engine, and the oil filter is the liver.

“Often overlooked, the oil filter is one of the single most important components of a truck’s engine,” say technical officials at Fram. “Its purpose is to prevent contamination from reaching the small gaps between the moving surfaces in the engine, as this can cause excessive wear and tear.”

“Essentially, while the oil itself cleans, cools and lubricates the engine, the oil filter does the same for the oil,” the officials explain. “A thin film, sometimes 20 microns or thinner, created by oil flow and pressure, helps protect bearings, piston rings and other moving surfaces.

“Quality oil filters, such as the ones Fram has developed for the past 70 years, trap many of the small particles that may potentially bridge the film gap and cause wear on the engine’s surfaces.”


Primary or “full flow” oil filters are intended to protect an engine from the “disastrous consequences” of relatively large (25 to 30 micron and larger) particles of soot, wear metals and other contaminants, say officials at the Technology & Maintenance Council (TMC), an organization whose mission is to improve transport equipment, its maintenance and maintenance management.

All of the oil circulated in an engine is pumped through primary filtration before it flows to the engine. Thus the term “full flow” filters.

For an engine equipped with secondary or “bypass” filters, a small portion of the oil - typically less than 10 percent - is diverted to the secondary filter, where it is cleaned and returned directly to the crankcase, bypassing the engine.

“Since only a portion of the oil is filtered during each pass through a filter, multiple passes are required to ensure that virtually all the oil is filtered,” TMC officials say. “With most systems, essentially 100 percent of the oil is filtered every 20 to 30 minutes of engine operation.”


“As engines advance with tighter clearances and longer oil-change intervals, oil filter design and technology must continually progress, accommodating even the subtlest refinements,” say Fram technical officials. “Filter efficiency has gradually improved to provide greater wear protection, with an emphasis on durable filter materials.

“Reinforced filter material ensures reliable performance during the entire change interval, no matter the length.”

Filters, such as the Fram Wearguard HD, were specifically designed for extended drain intervals, they add. And, the “use of synthetic oils in filters enables fleets to save money and resources by extending the time between drain intervals.”


Oil filters use either replaceable cartridges, elements containing fibrous media or containers to retain contaminants removed from the oil. The media is the filtering material in the oil filter element.

“Media construction and filter configuration are used to determine the filter’s efficiency for particle removal, its contaminant capacity and the pressure drop, or resistance to flow, through the filter,” explain officials at the Filter Manufacturers Council (FMC), which represents North American manufacturers of vehicular and industrial filtration products. “By working with these variables – efficiency, capacity, pressure drop – the filter’s performance level can be determined.”

Types of filter media range from mesh screens to depth style media, such as threads or chopped paper, to 100 percent natural cellulose to 100 percent man-made microfibers to almost any conceivable combination in between, they say.

The longer an engine oil filter is in service, the more debris it will hold. Consequently, filter cartridges or elements and containers must be replaced or cleaned periodically.

Eventually, the filter can become completely obstructed, causing the oil filter’s bypass valve to open. This value ensures continued engine lubrication by diverting the oil from the primary filter onto the engine.

However, allowing this unfiltered oil to the crankshaft, engine bearings and other precision components can result in damage to critical internal engine components.

“Generally, the larger the element for any given filter design and engine, the longer it may remain in service before it must be replaced, say TMC officials, who advise following engine and/or filter manufacturer suggestions for appropriate change intervals with extended oil drain intervals.


Oil filter “capacity” describes the amount of debris an oil filter can remove and hold before it becomes obstructed and causes the filter’s bypass valve to open, say officials at Purolator Filters. “When this happens, the filter allows unfiltered oil to reach critical internal engine components, which is better than no oil at all, but not much.

“When choosing a replacement oil filter, one should be certain that the filter has sufficient capacity to capture and hold all the debris it’s likely to encounter during its service life.”

Another feature to consider when choosing an oil filter is efficiency, add the officials. Efficiency is a measure of the percentage of particles of a given size a filter is able to capture.


“Because engines are constantly evolving to align with emission regulations, fuel economy requirements and other environmental needs, advancements in filter technology are essential,” say technical officials at Fram. “Potential changes may include equipping the lubrication system with lower-viscosity oils and reinforcing the filter to handle stress from increased EGR (exhaust gas recirculation), extended change intervals and eco-conscious designs.

“Future filter designs also will require increased efficiency to protect the intricate clearances within the engine cavity.”

In addition, the officials say bypass filtration may become more prominent if soot-loading parameters are increased. A few filters, such as the Fram Wearguard TRT filter, already are taking a more proactive approach by incorporating timed-release additives into the oil to help maintain oil quality and lengthen oil-change intervals.

“The industry also has seen an increase in permanently mounted housings that use replaceable cartridge filters,” the officials say. “Similarly, a transition from platform to cartridge filters is likely to address environmental and disposal issues.”