Great comments suggesting Leather Honey on this Reddit post about conditioning and preserving an old Swiss Army surplus pack. We love this one:
I swear by Leather Honey for restoring/maintaining my vintage leathers. Generally speaking, if I find a vintage leather jacket that fits, I buy it, so I have a few. My oldest is a bit over a decade younger than your pack there, but the stuff should still work for you. Made it shiny and pliable again, and even made the scratches and light cracking way less visible. It will likely darken the leather a bit, but then, the leather was probably darker when it was oiled properly, so there you have it.
Note that for something this old, especially as dried out as it looks, you would probably need more than one treatment to get the oils soaked through as much as you ought to.
*Edit Also, you can use it on pretty much anything. Boots? Leather honey. Jackets? Leather honey. Guitar strap? Leather honey. Belt? Leather honey. It will not hurt your leather. It would probably even help your skin if you suntanned too much for twenty years and have leathery skin. Maybe.