I'd try the scope mfgr first. Lacking any info there, perhaps one of the typical auto rubber "rejuvenators" such as Armorall, the various silicone-based products, etc. Any auto parts store or Amazon should have a selection. You don't want any to get to the internals or lenses, so perhaps one that is offered as wipes instead of in a spray bottle. Most of those work, albeit temporarily, to "re-blacken" rubber trim. But some leave a slick and/or sticky residue. I'd try on a small area before applying widely.