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Maintaining your filters is important!
Good quality filters changed regularly are one of the least expensive but most important maintenance items you need to keep track of on your car. A dirty oil filter can restrict your engine's vital oil flow. A clogged air filter can give bad gas mileage, and a missing or damaged air filter can let grit into your engine, scratching and scoring your cylinder walls!
Clean oil is fed to your engine under pressure by your oil pump. This oil is vital lubricant for your crankshaft, camshaft, piston rings, and other internal engine parts. If your oil pressure drops below 10-15 pounds, you engine will probably be damaged within seconds. "Throwing a rod", or a rod bearing failure, will result very quickly when oil pressure is lost.
A GOOD CLEAN OIL FILTER
In normal operation all your car's oil flows through the oil filter's filter medium. There are 2 types of oil filters: pleated paper, and fiber. Pleated paper filters have a special filter paper folded in a "W" pattern, wrapped in a circle. Oil flows in through a center core, through the filter paper, and out to the motor. Fiber filters have cotton or synthetic fber batting compressed between 2 layers of perforated metal. Oil flows from the inside to the outside on these filters also.
The difference between fiber and paper filters
It's hard to tell a fiber filter from a paper element filter from the outside; you have to cut it open to see the difference. In general, Purolator is the long-time fiber filter maker, and most everybody else is paper. Who's better? They're both good filters. The various filter maker's websites have cutaway pictures of their filters vs. other makers: it's worth a look.
Oil filter Bypass and Drainback
Oil filters also often incorporate a couple of valves: a bypass valve and an anti-drainback valve. The bypass valve is in case the filter clogs: it opens and allows oil to bypass the filter. In practice you should never need this valve: if it opens it basically means you've been so sorry about your oil changes that the filter is totally clogged. The anti-drainback valve prevents the oil in the engine and filter from draining back into the crankcase when the engine shuts off. This means the engine gets full oil flow and pressure quickly when it starts up again.