I pulled out a few half-used space pen refills that were sitting in the bottom of a box for maybe 15 years. Believe it or not, the darn things still worked.

For kits, though, I just toss in a couple of black (or red) medium Bics. Those things just won't die, even after years and years. The blue-ink jobs die a lot sooner. At 8c each from Staples, you can chop 'em down to your heart's content.