The all important BMW oil filter is one of the lowest cost auto parts you will have to regularly replace, but is also one of the most important. The appearance of the oil filter is a metal container with an opening at one end that has threads on it so it can be easily screwed into the engine. There should also be a polymer seal so the contact area with the engine can have a complete seal to keep the oil in and contaminants out. The inside of your oil filter made for your BMW has a paper element that the oil flows thru. The opening in this paper element in which the oil flows thru is between 3 and 5 microns. If the filter element become saturated with contaminates, there is a relief value in the oil filter that will bypass the filter element and allow for the oil to continue flowing thru the engine. If your oil is changed every 3,000 miles, this bypass valve should never need to be activated. All BMW oil filters are designed to remove contaminants from the engine oil. The main contaminant is carbon from the explosion of the fuel in the combustion chamber that leaks past the rings on the pistons and the valve seals. While it is a minute amount, it will still and always occur. To help keep the inside of your BMW engine as clean as possible the filter element should always be changed when the oil is replaced. All leaks on the engine should also be sealed so there is a minimal chance of dirt entering. The reason clean oil is so important of your BMW is to help prevent contamination from scoring the bearings and other moving parts of your engine. This way the engine will have a longer life span. Most oil filters cost less than $15 but save the owner of the vehicles thousands in repair costs.