You should certainly clean the filter from time to time Lee, if nothing else it's condition will give a guide as to the internal state of the overdrive. The filter sits on top of three magnetic washers, which should also be cleaned of ferromagnetic particles before reassembly. The best tool to remove the plug is a 'C' spanner of suitable size. In the absence of this you could unscrew the plug by gently tapping on one of the ears with a brass drift. Tim