Oil filter

Parts of an Oil Filter:
Oil filters are simple, but extremely effective. The main part of the filter is the filtration medium, which is usually paper or some form of synthetic wire-mesh used to capture particles as small as 10 microns. On the top of the filtration media is the Anti-Drainback valve, made of a silicone or rubber medium. This valve works as a one-way valve to prevent dirty oil from draining back into the engine.
On the bottom of the filter (the closed end of the steel can) is the bypass valve. Usually made out of a piece of plastic loaded with a spring. This valve is used so that oil can bypass the medium if it is too dirty, or if the engine oil is too cold and thick to pass through the filter. This valve is there to prevent oil starvation on very cold days, or for people who really like to push the limits of having their engine services regularly .

Of course, running unfiltered oil is just as bad as running without oil at all.

How the filter works:
Oil is fed directly to the filter from the oil pump, this is the first place that it goes. The oil enters one of the several holes that surround the center hole on the open end of the can. The oil then passes through the filtration medium (or the bypass valve) and is pushed out through the center of the can and into the metal tube that it is connected to. From there, it is fed to the rest of the engine.

When Installing a filter:
It is extremely important that you lubricate the outer gasket on the filter before you install it on the engine. The heating and cooling process of the engine will bind the filter tight to the engine block. After 3 to 4 thousand miles, it will become very hard to remove the filter if the gasket was not properly lubricated.

Life of a filter:
Filters are well abused when it comes to their duty in the engine. They're subjected to pressure upwards of 60 psi, have to filter all kinds of metal, dirt, sand, and other mess, and are typically not changed on time. The conventional oil filter is advertised to last 3 thousand miles. However, because Americans tend to be some of the laziest people on earth, many manufacturers have designed the filters to last around 5 thousand miles or more before they begin to give out. Depending on how hard you beat on your engine and how you drive it, the filter may clog sooner than you expect. Thus, 3 thousand miles is most always the best time to change your filter, regardless.

* Transitional Man adds "when changing oil filters on a racing or high performance engine it is a good idea to add fill filter with oil before installing. It's easy, just pour in, wait for it to settle, and repeat until the oil settles no more."

This is a tactic that I personally employ, as it keeps from pushing a bunch of air through the oil system and reduces engine wear on that initial startup after changing the oil.

The Japanese are REALLY bad about changing their oil on time. I've seen plenty of engines that died with under 30 thousand miles on them just because the oil wasn't changed on time.