While cleaning out my junk, I found an Antec Formula 5 thermal paste that was bought way back 2007-2008 (can't remember exactly when). It'll be handy to keep it, but is it still usable?
As long as it wasn't left open, I would bet that it should be fine. I still have a tube of arctic silver from 6 years that I use and performance doesn't seem any different from when it was purchased.