Thermal paste, Image via WikipediaQ: Can I use di-electric grease instead of thermal paste when I apply my heatsink? 

A: Dear God, no! Dielectric grease is the EXACT OPPOSITE of thermal paste when it comes to heat conduction! Instead of passing heat, it actually insulates heat! Dielectric grease are used in engine compartments to insulate spark plugs and some wiring connectors!

What does thermal paste do any way? Thermal paste is a thermal-conductive compound, sometimes called thermal grease. You put a little between the heatsink, and the CPU, and let the pressure spread it out to an ever coverage, filling in any 'gaps' between the two solid surfaces, so the heat transfer from the hot CPU surface to the heatsink is optimal.

Di-electric grease does NOT conduct heat. Thus, no heat will transfer to the heatsink, and the CPU will overheat.

So don't do that, even though they look similar.