I don't think you can repair the outer surface skin of the leather by any means...but cleaning them up and adding some dye will make the scuffs less noticeable. But once that outer layer of cowhide is damaged, there's no real way to get it smooth and shiny again.
Ok old fogges
I used the old try n true fiebings saddle soap on an older sling. I made sure afterwards it was dry. Dry on the front whipped it down again with a dry rag. Flipped it over it was bone dry on the back. I put one even coat of neatsfoot oil, rubbed it down.
Came back today, and there is white splotches in random places. So I took a dry cloth whipped it down, the white disappeared. Leather was dry feeling needed in my opinion another coat of neats. Did that, there’s not any standing oil on the sling at any time.
Is that white streaks dried soap? Mildew? Or what?
Salt.It might be some left over white fluid on your hand after you finished with the nude women's magazine.
Just kidding. I don't thing mildew would be white. I've been racking my brain on what it could be, but can't come up with an answer. But I'm old and my brain doesn't work very well all the time.