My very important laptop from 2001 is randomly turning off. I remove the heatsink, and the cpu (mobile Athlon XP 4) is shiny, and under the heatsink is this hard grey stuff. I cleaned the cpu and the bottom of the heatsink with alcohol. The grey substance (presumably thermal paste) didn't come off, so I don't know if this actually thermal paste or some form of thermal conductivity. So I want to buy some thermal grease, but I don't know if my laptop need it. I'll take a picture on request.
all cpu needs thermal paste to close the air gap between the surface cpu itself with the surface
of the heatsink. thermal paste to help absorption. heat to be
surface of the heatsink. if not
using thermal paste will occur the air gap, ... so that hot
can't absorbed maximally . therefore you
need a good thermal paste like AS5 and so on.
My guess is that your original heat sink was affixed with a "thermal pad" which is a bit harder than paste to remove. The gray substance should come off with isoprophyl alcohol. May take some soaking and rubbing with a plastic scrubber (aka "Dobbie Pad"). The purpose of thermal paste is to aid in the transmission of heat from the CPU surface to the heat sink primarily through the elimination of the air boundary layer between the two.
NOTE: Some thermal pastes (i.e AS5) have curing issues whereby they will not reach their full potential until after 200 hours of "thermal cycling". Some reviewers have estimated that this can the best pastes stake up to a year "under normal usage". Shint Etsu provides them best of both worlds as it matches the best of the best in ultimate performance but does so the same day you put it on.