The ultimate survival 'pen' is still a pencil. Little known fact: A pencil, as long as it is gently sharpened, will write more pages and last longer than any pen. Pencils are usually lighter, they never freeze or leak, they are still usable if broken and they provide a ready supply of tinder if your in a bind.

I have also used the graphite in a pencil to keep a firearm going after it gummed up from dust and sand. I removed the bolt assembly, wiped everything dry and spent some time rubbing the side of the pencil point on the pieces until they developed a shine from the graphite. Put back in the mechanism worked smoothly and shrugged off the dirt.

That said, I usually have a pen in my pack. But I always have a pencil. Don't look down on the simple reliability of a pencil.