If you push on the plunger and nothing will come out.
Don't use it!
Sorry OP, couldn't resist.
AS5 from 6 yrs ago will still be good as long as it was capped and sealed, however most do not know that the AS5 today is an improved formula than what was on the market 6 yrs ago.
That information I attained from an Artic Silver company representative, when I was doing the Thermal Roundup.