Automobile oil filters look simple, but they are the subject of continuing research and development to make them work better. Physically, spin-on oil filter resembles a metal can that houses varying types of filter media. These are the materials that capture organic or inorganic contaminants as oil flows through. Organic contaminants include bacteria and other organisms that form gross sludge. Inorganic contaminants consist of dust that is ingested into the engine, along with trace amounts of wear metals from bearings and other internal parts. The oil enters the oil filter under pressure through the holes on the perimeter of the base plate. The dirty oil then passes through the filter media where it is cleaned. It then flows to the central tube and back into the engine through the usually threaded hollow center mounting stud. Early designs used steel wool, wire meshes and metal screens. Latter, bulk cotton or various woven fabrics like linen were used. When disposable filters became popular, cellulose and papers were used to minimize production costs. Finally synthetic media oil filters were introduced where special man-made fibers are utilized. Fiberglass and metal fabrics are also sometimes used for oil filtration. Today, most low-cost disposable spin-on oil filters use cellulose filter media. Better quality oil filters use synthetic media, while top end oil filter use micro glass or extremely fine metal mesh.