Op here, to further explain what I mean an analogy: fry a steak in a pan and afterward that grease leftover is what I am refering to. You can pour it out (ala change oil), but a layer still clings to the pan however long you let in drain. Now grab a clean hand full of crisco and rub in pan (new oil) and this will clean the residue, mostly by diluting it. You can easily wipe most of this stuff off, and get a nearly clean pan. Add a little soap, which I try via a little flush into oil pan, to try to remove this last and still present otherwise contaminant to the new oil. You can agitate via several pours, spraying, and even bouncing car up/down by pushing on it.