To start, let me say all my milsurp slings are just slings purchased mostly used from Liberty, Sarco and/or Numrich (so no purist blasphemy committed) and I am not a leather worker at all. That said, if your into preserving the leather, but maintaining a functional sling then it needs to be cleaned, oil and preserved. If you where ever in the military and had to shine your boots, its more so the same process. For me, I accomplish the following to leather slings, frogs and ammo pouches:
- Clean the leather with a gentle leather cleaner (mineral spirts if its really dirty though some would frown).
- Clean any brass or metal.
- Depending on the leather and how it came rub/wipe it down with neatsfoot oil (again 20+ years I've had the same bottle).
- Spray/Wipe down the leather with Ballistol, let it soak then normally it does not require a wipe down but if needed remove any residual oil.
- Wipe in a leather weather/water proof paste (Kiwi makes some, I've had the same can for 20 + years and its still good).
- Buff with a soft shoe brush.
This process above works with leather that is not to dried out. For leather that is dried out, it requires reapplication of leather oil (Ballistol, neatsfoot or your choice) and repeat until the leather seems to be not as stiff. After the leather has soaked up the oil, then you can put the weather/waterproof paste wax on it and buff. Don't forget the underside of the sling as well.
Again the process above works for me with what I have on-hand. I agree with DannyBMW to roll them up in a Ziploc bag, but I would treat them in some for or fashion as I described above before placing them in storage. Also, depending on the item you might not want it so soft that it collapses from its form or shape (e.g. ammo pouches).
Last it's satisfying to take a $7 frog or a $9 ammo pouch and restore it to a "new" or "reconditioned" look. That's just me I'm into taking sloppy seconds and making them first choice items LOL.
One note, be careful with the oils etc. you use as they may change the color of the item your treating.
Regards.
Scot