MX-2 is generally a better performer and not even slightly capacitive. It tends to perform better than MX-3 and 4 and suffers less long term deterioration. MX-2 is one of my all time favs
Tom's Hardware's thermal paste roundup results disagree with that. MX-2 is better only when used on the GPU. For the CPU, MX-4 is better and MX-2 and AS5 are about the same.
Closed-Loop Liquid Cooler; High Mounting Pressure
MX-4 32.4
MX-2 32.9
AS5 33.0
-> MX-4 is better than MX-2 which is about the same as AS5
CPU Air Cooler; High Mounting Pressure
MX-4 36.5
MX-2 36.6
AS5 37.7
-> MX-4 and MX-2 are about the same, AS5 is worse
CPU Air Cooler; Low Mounting Pressure
MX-4 36.9
MX-2 37.5
AS5 37.7
-> MX-4 is better than MX-2 which is about the same as AS5
Air-Cooled GPU; Medium Mounting Pressure
MX-2 66.5
MX-4 67.5
AS5 69.5
-> MX-2 beats MX-4 beats AS5
One can wonder about the margin of error though. Each application of thermal paste is a little different, each mounting a little different, there's a good chance that those decimals are pretty meaningless unless a sufficient number of reapplications was done, but I don't see any mention of testing methodology in the roundup.
TweakTown's results: MX-3 beats MX-4 beats MX-2 beats AS5. However other sites have found that MX-4 beats MX-3 (
hardwareheaven,
eTeknix), so I'm inclined to believe that MX-4 is indeed the best of the bunch for CPU cooling (as you would expect, given that it's then newest iteration in the MX series).