K5 (or thermal pads) is ONLY needed between the copper pipe and metal shield plate. It is a pad replacement compound and is not meant for high heat applications like CPUs as it boils and creates air gaps. Air=bad for heat transfer.

Regular thermal compound/paste should be used on the CPU. You’ll have a sticky mess to clean but if you want proper cooling it needs to be done. The instructions clearly state that regular compound is used on the CPU.