You want to use as little as possible. The "mayonnaise between the bread and meat in a sandwich" model of applying thermal paste is wrong. The majority of the heat transfer is via direct metal-on-metal contact between the processor and the heat sink. If you apply a thick mayonnaise-like layer between the two, it's actually worse than having no thermal paste. The role of the paste is to fill in microscopic air gaps between the two surfaces because they aren't perfectly flat. Not to sit between the two surfaces. Paste conducts heat about two orders of magnitude better than air. But likewise, metal-on-metal conducts heat about two orders of magnitude better than paste. So having paste reducing metal-on-metal contact can easily...