Hello m0nk3n
I'm surprised your pc even boots with that ram configuration. I would remove the 4x2GB kit and just use the 4x4GB kit.
Thermal paste in the cpu socket is not good, it could be causing the cpu to not make contact with some pins as to why it's only reading 18GB. You could try getting it out the best you can with maybe a toothpick being extraordinarily careful. If it's non conductive thermal paste that's a plus but that's still a scary predicament.
If cpu-z is showing 24GB then you may not even have any problems.
You should really be running 16GB, 32GB, or 64GB.