I still dont get it. If the oil is sucked from the pan and put in the HPOP res then passed through the pump and injectors then the oil returns to the pan. the "top" end is not a closed system. that oil leaves the "top" gets to the bottom then mixes with the 15 qts there then gets fed to the top again. now if it takes 5000 miles to empty what is there, then yes i agree, it should be changed. anyone know how often the "top" oil circulates?