I just purchased an '03, and last night discovered the HOAT antifreeze requirement. Tell me a little more about this coolant filter. Is there a factory coolant filter on the '03 model, or are you speaking of a heavy duty truck? Also, where is HOAT antifreeze avaliable, I went to the store last night and read the backs of all the bottles and none stated that it was HOAT compliant, therefore I came home empty handed and referred to the TDR. Thanks for any and all help!!!