high purity isopropyl alcohol and a lint free cloth. possibly an old toothbrush to clean the pits in the heatsink.
"Arctic Silver 5 was formulated to conduct heat, not electricity.
(While much safer than electrically conductive silver and copper greases, Arctic Silver 5 should be kept away from electrical traces, pins, and leads. While it is not electrically conductive, the compound is very slightly capacitive and could potentially cause problems if it bridges two close-proximity electrical paths.)"
better use arctic silver ceramique instead its 100% non-conductive and theres barlely any temperature diffrences between various thermal pastes
thats like saying you shouldn't walk across a bridge because it might collapse. the only reason that message is there is as a disclaimer to release arctic silver from liability. you would probably actually have to try your hardest to fry something with AS5