A portable apparatus for sensing position of an oil port on a rotating element (e.g. the wheel or track of a heavy vehicle) has a position sensor in electrical communication with a wireless transmitter. The wireless transmitter is configured to receive signals from the position sensor and to send signals to a wireless receiver. The apparatus further has a mounting structure (e.g. a magnet) for temporarily mounting the apparatus on the rotating element. An indicium on the apparatus associated with a desired service action to be performed on the rotating element correlates an angular position of the oil port to a pre-determined reference position of the position sensor, the angular position of the oil port being a correct position for performing the desired service action. In use, the apparatus is mounted on the rotating element so that the indicium points at the oil port and then the rotating element is rotated until the receiver indicates that the oil port is in the correct position.