On a processor/heatsink combination? Ive seen different adhesive pads.
Really you just want to help provide more intimate contact. Something soft with a bit of give. A grease would be best, vice a solid, IMO. I wonder what the thermal conductivity of a moly-fortified grease is.
Whatever it is, remember, less is more.
I might use copper or Nickel antiseize. Clean it well before using the right stuff.