I guess it depends on what you use. Thermal greases have an oil base so they stay soft, since dried out thermal interface material is a bad thing. I just bought a thermal pad from amazon for my Kraton's heat sink to motor interface. I've used it extensively in modded PCs and some electronics projects. Get the grey stuff, never the blue stuff, and order the thickness you need. It's a little thin and fiddly in some applications, but if you have even surfaces to work with, it just goes in like a gasket between mating parts.
