How do oil filters work?
Early internal combustion engines did not use oil filters and, coupled with the poor quality of oil available at the time, vehicles required frequent oil changes. Eventually, the first full-flow oil filtration system was developed. Basically, this arrangement allowed for the oil to flow through the filter before it reached the critical working components inside the engine.
So far so good, but there was (and still is) a big caveat: The vast majority of pressurized lubrication systems found in internal combustion engines incorporate some form of filter by-pass to protect the engine from starvation under certain circumstances. A good example is a very cold weather. In this situation, if the oil is too thick, it is allowed to bypass the filter. Oil can also bypass the filter when the filter is plugged. Because of these events, oil is sometimes not filtered, even when the engine is fitted with a full-flow oil filter.
In operation, oil enters the oil filter through a series of small holes on the outer edge of the base flange. The oil is then directed through the filter, eventually making an exit into the engine through the large center hole. Most modern oil filters are equipped with an anti-drain-back valve. This is often some form of rubber membrane that covers the perimeter holes in the base flange. The membrane is forced aside as oil enters the filter case. When the engine is not running, the rubber membrane covers the holes. Obviously, the anti-drain back valves maintain oil within the filter. In turn, they prevent engine dry starts (when the engine is started with no oil).