[GU]elmur_fud
I have balls of Depleted Uranium
Does this stuff go off ? I have a tube from years ago and it seems to have gone slightly gooey unless mistaken. Last time I did a cpu I could properly ease it across the surface with a plastic card.
Spoil? Not as such. They probably shouldn't be frozen though. At some point the materials will separate and you will get an oil and a more hard crumbly substance with the other chemicals. With regular bearing grease this occurs at around -80°F (If my memory is correct) but I haven't even the slightest clue what low temp would compromise thermal grease. Since it isn't intended for cold temps it is entirely possible that it wouldn't be very tolerant.
More likely though since some types of thermal past use chemicals that can easily evaporate over time (they are in fact solvents and designed to do just this) if you break their seal it is only a matter of time before whatever unused portion reaches optimum operating consistency due to evaporation.
Also I'd take this opportunity to note that you should be careful to not get any of the metal based compounds on any circuitry as the metal based are electrically conductive while conversely the non-metallic are insulative.