Fastening method, assembly and structure of engine oil fill extension tube can include an oil fill extension tube and a self-locking washer. The oil fill extension tube can include a plurality of locking edges disposed towards a first distal end of the oil fill extension tube, where the plurality of locking edges can define boundaries of a plurality of slots. The self-locking washer can include a plurality of teeth disposed on its top surface. The self-locking washer can be oriented over an opening in the oil pan, after which the oil fill extension tube can be threaded into the opening. The plurality of slots and plurality of teeth can be ratcheted together to fasten the oil fill extension tube into the oil pan when the oil fill extension tube is rotated clockwise. The washer can further include a locking extension to securely engage a portion of the oil pan.