A method for purifying an oil from contaminating particles in a centrifugal separator with improved separation continuity by means of a separation aid which is of higher density than the oil and which binds said particles, wherein the centrifugal separator comprises a rotor body caused to rotate about an axis of rotation and a screw conveyor; said rotor body comprising a cylindrical upper portion connected to a conical lower portion, said upper and lower portions (3,5) delimiting a separation chamber; and the screw conveyor arranged in the rotor body and rotatable about the axis of rotation at a speed which differs from the rotation speed of the rotor body, the screw conveyor comprising a central core, a sleeve part and at least one conveying thread, wherein: the central core extends axially throughout the whole of the lower rotor portion; the sleeve part, which includes a plurality of apertures distributed around the axis of rotation, extends axially from an upper portion of the a screw conveyor to conical portion; a plurality of wings distributed around the axis connect the central core to a central sleeve within the sleeve part, the central sleeve includes a conical portion and a lower support plate; the at least one conveying thread extends in a screw-like manner along the whole inside of the rotor body and is connected to the sleeve part and the core; the oil which is to be cleaned is discharged from an inlet within the central sleeve, passes through a passage between the core and lower support plate, the passage is in communication with the inside of the rotor body via channels between the wings so that the oil is, after discharge, led to the upper portion without passing the at least one conveying thread, separated particles are discharged via a first outlet and the oil cleared of said particles is discharged via a second outlet, said oil being mixed with an amount of separation aid, being supplied to the separation chamber and being purified therein by the particles being bound to said separation aid, which separation aid is forced out by the rotation of the rotor body to the periphery of the rotor body, a small flow of separation aid and particles bound thereto being discharged from the separation chamber via the first outlet, and a flow of purified oil being discharged from the separation chamber via the second outlet, wherein said separation aid and particles bound thereto are conveyed along the inside of the rotor body by the at least one conveying thread towards and out through the first outlet and that the oil which passes via the second outlet passes, before that, through intermediate spaces formed between truncated conical separating discs arranged in a stack in the upper portion, wherein the conical separation discs have base ends facing away or towards the outlet for separated particles and are held together axially between an upper conical support plate and the lower conical support plate by the central sleeve, which extends through the stack of truncated conical separating discs. said rotor body comprising a cylindrical upper portion connected to a conical lower portion, said upper and lower portions (3,5) delimiting a separation chamber; and the screw conveyor arranged in the rotor body and rotatable about the axis of rotation at a speed which differs from the rotation speed of the rotor body, the screw conveyor comprising a central core, a sleeve part and at least one conveying thread, wherein: the central core extends axially throughout the whole of the lower rotor portion; the sleeve part, which includes a plurality of apertures distributed around the axis of rotation, extends axially from an upper portion of the a screw conveyor to conical portion; a plurality of wings distributed around the axis connect the central core to a central sleeve within the sleeve part, the central sleeve includes a conical portion and a lower support plate; the at least one conveying thread extends in a screw-like manner along the whole inside of the rotor body and is connected to the sleeve part and the core; the central core extends axially throughout the whole of the lower rotor portion; the sleeve part, which includes a plurality of apertures distributed around the axis of rotation, extends axially from an upper portion of the a screw conveyor to conical portion; a plurality of wings distributed around the axis connect the central core to a central sleeve within the sleeve part, the central sleeve includes a conical portion and a lower support plate; the at least one conveying thread extends in a screw-like manner along the whole inside of the rotor body and is connected to the sleeve part and the core;