The engine oil filter is responsible for trapping and storing debris and other contaminants in the lubrication system that are generated as the engine runs. The filter prevents wear of moving engine parts such as engine bearings, pistons, and chains.
Engines generate debris as they run. There is a lot of moving metal in there, and even with good lubrication, a little bit of wear occurs. This wear material needs somewhere to go where it won’t interfere with proper lubrication. The filter’s job is to catch it.
The consequences of this wear can include increased oil consumption, loss of oil pressure, and loss of lubrication resulting in engine failure. Many of these internal engine components are costly to service or replace. Debris is always a consideration because engines generate debris as they operate (very slowly if the engine is healthy and well-lubricated). So slowly in fact that many engines can run for 300-400k miles before requiring rebuilding or replacement. But this debris requires the use of a filter to trap it and store it so that it can’t continue to circulate through the engine.
This means that we want filters that are good at catching debris and have enough capacity to store it until the next oil change comes due.
Due to the critical responsibility of the oil filter to both capture and store debris, you want a filter with high efficiency (it does a great job of catching debris) and good capacity (it has room to store all trapped debris until the next oil change comes due.) A high efficiency filter will trap more debris which makes it storage capacity that much more critical so that it never has to go into bypass mode (please see below.)
low efficiency: A low efficiency oil filter will allow debris to pass through and continue to circulate through engine bearings. This increases the odds of premature bearing failure.
A low capacity oil filter is at risk of going into bypass before the oil is changed again. This refers to the condition in which the filter is so full of debris that it must go into bypass to allow engine oil to keep circulating. While this is better than losing oil pressure, it allows uncontrolled circulation of debris within the lubrication system!
A sub-par (or wrong application) oil filter may run out of storage capacity before the next oil change comes due. This will result in what’s called “bypass.” In this condition, the filter still allows oil to flow, but debris bypasses the filter and remains in circulation through the engine.
Filters with low quality seals or poor thread cuts are at risk of working loose, which can cause rapid oil & lubrication loss followed by engine failure. Filters with poor (or missing) drainback valves lengthen the time required for oil pressure to build at startup, which increases the rate at which the engine wears. (Again, all engines wear as they run, but in good conditions, this process is slow. Much of this wear occurs during start-up when there is no oil pressure.)
Filters are often not fully tightened, and they’re not even fully tightened sometimes when installed by a repair facility. Do they usually work loose? No. But if they do work loose, the consequences can be quite severe. A good rule of thumb in many cases is 3/4 to a full turn past the zero clearance point during installation. In most cases, removal tools will then be required to get the filter back off. This is normal and does not indicate an over-tightened filter.
99% efficient means nothing without knowing what particle size is applicable. To put this in a humorous way, all filters are 100% efficient at stopping “tennis balls” from circulating through your engine, but what about particles with a 20 micron size? Look for ratings that tie efficiency to particle size.
A common myth is that oil filters should be removable by hand. In fact, many spin-on filters require a 3/4 to full turn past zero to be fully tightened, and filter tools are commonly needed both to fully tighten and then loosen these filters. Tightening them less risks the filter working loose during engine operation and resulting rapid oil loss.