Re: How do you soften and protect leather slings?
strongly suggest obenaufs as mentioned above.
a) leather is NOT skin - it is tanned, dehydrated, chemically prepared organic material that is vastly different than skin, which exfoliates, self-repairs, sweats, breathes, bleeds, etc.
b) neatsfoot oils and other natural oils rot (oxidize) like olive oil or other natural oils. This will, in turn, aide in your leather rotting prematurely. You know what the "tanning" process is? It is taking OUT the natural oils in the hide so it doesn't decompose/rot. As pointed out earlier, you can use a little of these oils if you use them sparingly, and it won't seem like it is hurting anything if you keep your leather sling dry and clean 99% of the time anyway.
c) I don't own any stock in Obenaufs, but I lived in the Pacific Northwest for 30 years and wore boots outdoors in the rain six days a week for 20 of those 30 years. And it is the best product I've ever used - and I've used everything mentioned in this thread except hand lotion: vaseline, other petroleum jelly, neatsfoot, mink oil, leather fats, all of it.
But if it is a leather sling, it is probably not meant for bad weather anyway. And a little bit of whatever you decide won't hurt anything too badly to notice...