Have you ever watched someone make a sandbox in a playground? When it comes time to put the sand in, they use a sieve to sift the sand. The sieve is a metal grate or screen with very tiny holes to let the sand pass through, but it will catch rocks, glass or any other large object that is unwanted in a child's sandbox. As objects fill up the holes in the sieve, the sand particles that can slip through get smaller in size and some larger sand particles get trapped in the sieve too. If the sieve gets clogged full, it won't let any sand pass through, and it will then need to be emptied.

To the left is a picture of how an oil filter operates. The contaminants in an engine are caught by the oil and carried to the oil filter. The function of the oil filter is to efficiently remove and retain those contaminants so that they cannot work their way into other, sensitive engine parts which can be ruined by even tiny amounts of dirt. The oil filter must do this job without offering too much resistance to the flow of oil through the engine.

The oil filter is composed of a housing (that looks like a metal can), and a filter cartridge. The filtering material in this cartridge can hold a remarkable amount of dirt and sludge, but at some point it loses its ability to trap any more dirt. As with the motor oil, the oil filter becomes less effective and fills up with trapped contaminants faster, as the stress put upon the engine increases. In the illustration above, if the sieve clogged up, becoming full, and you put more sand into if the sand would simply pile up and spill over the sides.

It would be disastrous if an oil filter became so filled with dirt that it became plugged and no oil would reach the engine. The damage would be immediate and devastating. In an oil filter, when it becomes fully clogged with contaminants, it doesn't start spitting oil out the sides of itself, but it has a bypass valve to let the oil go around the filter element and back into the engine unfiltered, since no more is able to flow through it. Obviously, this is better than having the engine blow up, but it is not a good idea to have unfiltered oil circulating through the engine, either. This condition slowly starts somewhere around 4,500 to 5,500 miles of oil filter use, after the last filter change. So, it is wise to change the filter often enough so that the relief valve never has a chance to open.

Up until the fifties, most vehicles used a "bypass" filtration system, using a permanent filter housing with replaceable filter elements. This system pumped only about one tenth of the oil at a time through the filter, the rest of the oil went back into the engine unfiltered, This helped keep oil compression high, and took some of the strain off the oil pump. But it also meant that fewer contaminants were removed from the oil, and engine wear was higher. Bypass filtration is still used today on some industrial and commercial applications as a secondary "helper" filtration system which runs in conjunction with a full-flaw system. As the demands put on engines became greater, more and more vehicles began to use the "full-flow" filtration system, which pumps all of the oil through the oil filter. In other words, not a single drop of oil is ever circulated through the engine more than once without being filtered. Obviously, this leads to cleaner oil and reduced engine wear. In most cases the full-flow filtration system uses a throw-away filter that is comprised of the filter element and the housing. There are a few which retain the housing and dispose of the filtering element. Many of today's oil filters have an anti-drain back valve which closes whenever the engine is shut off. This valve keeps unfiltered oil from draining back into the engine. When the engine is started again, the valve reopens. This feature helps in start-up by holding that extra oil, and being ready sooner with oil, to pump through the engine (instead of waiting for it to make its way from the drain pan, through the filter and into the engine).