If in doubt use some thermal paste.
DON'T take chances with a CPU heatsink!
($2 paste, or melt a $200 cpu, your choice)
Radio Shack carries thermal compound for heat sinks.
If you get lucky a local electronics/computer store may have some artic silver, but the Radio Shack paste will work fine.
Personally, i scrape those pads off and use thermal paste instead.