Arctic Silver is the most often used/suggested brand of thermal paste.
I use it on my own builds and it has reduced the CPU temp by several degrees.
Arctic Silver also sells a cleaning kit to safely clean off the old thermal paste from the CPU and heatsink and prepare the surfaces for the new thermal paste.
https://www.newegg.com/p/pl?d=arctic+silver&N=100007788&name=Thermal%20Compound%20%2f%20Grease
