Method for preventing biofilm formation on at least one luminal surface of a catheter system (1). The catheter system (1) comprises a collection vessel (4) and a tube (3) coupled to the collection vessel (4). The catheter system (1) comprises a catheter connector (2, 20, 200) coupled to the tube (3) and connectable to a catheter (6). The method comprises the consecutive steps of connecting the catheter connector (2, 20, 200) to the catheter (6) and supplying a silicon oil to an interior (25, 205) of the catheter connector. A catheter assembly comprising a catheter connector (20) comprising a port (24) providing access to the interior (25) of the connector and a dispensing unit (50) containing a silicon oil. A catheter connector (200) comprising a reservoir (207) containing a silicon oil and a releasing mechanism (208) releasing the silicone oil into the interior (205) of the connector.