If you're certain they're perfectly flat and mirror finishes, you can try not using any paste. Every extra layer reduces heat transfer, and the point of paste is to fill up those little gaps that might contain air. If you have the time, I would really suggest trying both with and without paste - and let us know the results!
I don't know if you've used or even heard of gauge blocks, but they're
extremely tight tolerance, perfectly finished blocks for exact precision measurements to calibrate metrological equipment. Anyway, my point is, the blocks are so precise that you can literally stick them together by pushing down and a slight twist (start at 90* off, then twist with pressure till they're parallel) and they hold because they completely force the air out. Perfect surfaces.
"Wringing gage blocks together, which means combining them end-to-end to add their measurement values, is the real key to accurate gage-block setup and use, and it's also the most misunderstood concept. The phenomenon of wringing gage blocks so they seem to "stick together" occurs for two reasons. First, there is an adhesive action because of an ultra-thin film of oil or moisture between the blocks. Second, there is a molecular attraction or bonding, between the blocks because of the very flat and parallel mating surfaces. That's why the better the block geometry, the better the wring will be."
So anyway, based on that theory, if you really went to a fine polish then no paste might be the way to go. I guess it just depends on how far you went. You can get a hazzy reflection with only 1000 grit paper, and in that case, use some paste
And my recommendation would be Arctic Silver, I've read good things and also at the local PC store, they said "want the best stuff?" I said "yea, Arctic Silver" and they said "Of course, it's the only one I recommend" lol.