I've been told that I'll have problems with the fans if I don't use the disk that came with the computer.
You've been told wrong.
Reinstall away, but it likely won't help the temp variation (I hope that I am wrong, but....).
Even without leaks, LCS get gummy and lose effectiveness.
Wouldn't the full box set of Mac OS X 10.5 have all the optimised tools for the platform on which it's installed?
I'm happy to go in an re-apply the thermal interface,
Never a bad idea with thermal compound that is over 5 years old.
do you mean that the coolant liquid is not moving in the LCS system?
Not moving well, yes.
A long but fact filled post with a full rebuild included: