When it absolutely needs to stay on forever, Arctic Silver's ASTA-7G is the way to go. This permanent epoxy is made with 99.8% pure micronized silver and has a silver content of 62-65% by weight.
Arctic Silver is designed to conduct heat and not electricity even with its high silver content. When cured it does have some capacitance however, so it's best to keep it away from traces or leads. Arctic Silver conducts heat away from sensitive components and improves the efficiency of your cooling system. Keep things cool and functioning under the hottest action!