I have obtained an unopened, sealed, boxed CPU dating back to 2009. It’s an Intel Core 2 Duo E8600 with heatsink + thermal compound pre-applied. Is the stock thermal compound still OK to use after five years on the shelf?
It should be, given that in the same amount of time, it could've been attached to a CPU and has to enure that as well.
If you don't trust it, just scrape it off and apply fresh coolingpaste (which is different than those strips. The paste can actually dry out.
Journeyman Geek points out that if its slightly hard it should be ok, if it crumbles like dried toothpaste, you need a fresh coat of the stuff.