I've read a couple of different comparisons between various types of thermal paste, and from what I've seen it doesn't seem to really matter (performance wise) which thermal paste you use as long as you use some form of thermal paste. That being said, you always want to use some form of thermal interface, and I've found that higher quality ones tend to last longer.
With thermal paste you only need a paper-thin layer for it to do its job. Essentially what it does is fill in the tiny imperfections on the heat spreader and the heat sink so that they make better contact and can transfer heat away more efficiently. If you don't use some form of thermal interface (paste, pad, adhesive, etc) you basically end up with tiny pockets of air in the microscopic pits and groves on the heatspreader and heatsink.
If you really want the most efficient air-cooling you can get from your parts, you could try lapping the heat spreader and heatsink. To do that, get a couple sheets of very fine sandpaper (1000, 1200, and 1500 or 2000 grit) and a flat piece of glass. Starting with the lowest grit, lay it on the glass and then gently slide the heatspreader and heatsink back and forth over it for a minute or two, then move up to the next grit. By laying the sandpaper on the sheet of glass you ensure you have a flat work surface so that you don't accidentally sand the heatsinks unevenly.
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