^ Well I wouldn't try it, except maybe on an old CPU I no longer cared about. Or if I underclocked the CPU until I could get some decent thermal paste.
Thermal paste is formulated to maximize heat conductivity (i.e., low thermal resistance) and to fill in the micro-gaps & voids between the IHS and the cooler base, and I guess not develop voids as it dries out.
But if you enjoy a fresh, minty smell from your CPU, then I guess toothpaste might do the trick
.