Ive learned that my '02 malibu with 24K has no ATF drain plug or dipstick. The owners manual says under normal usage that no change is needed. Under heavy usage every 50K a flush is needed. So, Im thinking I wait until about 50K, then have the 9.5 quarts flushed and filled with Amsoil or Mobil 1 or something long lasting. Then if they car makes it to 100K while I still have it, another flush with synthetic. Is this reasonable or too long (50K). Should I try to get some auto RX in there before its flushed?