Copperkote is not a cement. It does not dry out and does not glue the
cpu to the heatsink.
Actually there is insufficient data to determine this.
Darn near anything will suffice on old CPUs with their lower
heat density, but no long-term use can have happened yet on
high thermal density cores such as Athlon XP, simply because
they haven't been in the market long enough. Being a
suspension of larger copper particles in petroleum base, it
is expected that it would tend to separate and pump out
after numerous thermal cycles. Perhaps there's a newer type
of Copper Kote with ester base, but it wouldn't need be
formulated for CPUs, merely the lowest cost per the spec'd
use.
On the other hand, P4 and Athlon 64 with their integral
heat-spreaders, will again allow use of lower grade thermal
interface, relatively speaking.