Electrospun polyacrylonitrile (PAN) fibrous membranes with fiber diameters in the range of 120-250 nm were prepared and employed as a membrane material for lubricating oil filtration due to their excellent chemical and thermal resistance as well as high oil wettability. They separated different types of carbon black with varied separation performances, depending on the size of carbon black. The smaller the particle size, the higher was the average flux. As the content of particles were extremely high, they were able to pack closely together on the membrane surface giving a so called “layer effect” and thus reducing the effective pore-size of the electrospun membrane significantly at the surface. This dense “cake” acted as the separating layer for the electrospun membrane, resulting in unusually high recovery flux for high amount of particles. The fouling mainly occurred within the surface of the electrospun membrane since particles were mostly retained on the surface and only slightly within and at the bottom of the membrane.