SOME CM400 DO NOT HAVE OIL FILTERS. DON;T ASK ME WHY. IF IT DOES HAVE A OIL FILTER ITS EASY TO FIND. IT WILL BE LOCATED ON THE BOTTOM SIDE OF THE ENGINE. ALL YOU NEED TO IS DRAIN THE OIL, THEN USE A SOCKET TO REMOVE THE FILTER. MAKE CERTAIN THAT YOU HAVE A NEW FILTER BEFORE YOU REMOVE THE OLD ONE. THE NEW FILTER WILL COME WITH A NEW O-RING THAT IS NEEDED TO ENSURE A GOOD SEAL..