Oil filter removes sludge and other dust particles from engine oil protecting the engine from excessive wear and tear. It is usually made of pleated paper. One of the important purpose Oil Filter serves is when the car is turned off, the oil is drained to the bottom of the engine. This leaves the top parts of the engine without any lubrication. Oil filters retain the oil with the help of its anti drain back valve and when the car is turned on, Oil filter instantly supplies its retained oil to the top parts of the engine lubricating them properly. The oil filter should not offer much resistance to the flow or else the car won't start on a cold day. Oil filters should be replaced at regular intervals as per the car servicing. A clogged oil filter will restrict the oil flow to the engine decreasing its efficiency. Generally as per Mechanic's the oil filter should be changed every 5000km or with the change in engine oil.