ok i think i found some helpfull info. if i understand it correctly it means paste being thicker than grease will fill in better and therby cool better. also the higher the silver content of the paste the better the cooling. ok so i guess that means grease is ok but paste is better. it also said nothing about ever having to replace it so i believe that is bad info. it did say it needed break in period to cool at room temp to set properly so if you leave computer on all the time you need to turn it off a few times to let the paste set. the cooling helps it set.
this link was were i read about it, it is artic silver review.
here is the link to applying the paste where the break in period is explained.