I use WD40 on a cloth, to get the majority off, then another wipe with a clean section of the cloth and more WD40, then swap to a wipe with the alcohol and a clean cloth to get the oil off. Then I use some silicone oil and apply a thin film to the surfaces, and then a thin blob of heatsink compound spread over the top, then apply the heatsink. After all, you do not need all the old stuff off, it has has to be removed from the active surfaces.