I use Neatsfoot Oil. But you have to get pure Neatsfoot Oil, not "Neatsfoot Oil Compound." The compound is only partially Neatsfoot Oil, with a bunch of thinners and carriers thrown into the mix.
Neatsfoot Oil will usually darken and soften the leather, which is generally not a problem for a sling but not what you would want for a holster that was boned or semi-boned to fit the gun.