NIB and unstarted
Assuming the guy was legit, wouldn't the warranty(s) been in his name, and starting the moment of the sale? The bigger saws are 90 days for everyone, the 390 and down is a year -or 90 days for a pro user.
As for the ungassed, NIB specimens... how old is too old? Stihl fires each one up at the factories before going in zee box, yah? So they all got at least one "wake-up" shot.
I guess if one could store one in a chamber with inert gas it could last forever... (Instead of a lame old fish tank, have The Saw Tank! )
In the real world, how long will seals and the other rubber bits of a brand new piece of eq last if stored away in a climate controlled room? Or a non-CC shed/shop? Cased and out of sunlight either way.
I assume one should crank it over from time to time, but should one add & gyrate a few cc of raw UltraHP (or other goo?) once or once & awhile? I know that wouldn't be "NIB" anymore, but if it meant a runner in a dozen years, that's the ticket.