I will suggest silicone, not any lube that has petroleum in it.
Rubber hardens due to exposure to sun (and petroleum products over time). In it's natural state rubber is white (as why old tires were white). anyone remember those movies in school of the natives pouring latex and rolling big donut wheels of rubber?
the black comes from carbon black added to uv protect the rubber from oxidizing. it still will over time but silicone is the by-product of rubber oxidizing. wind dirt and ran carries it away, the silicone protects and rejuvenates to top few layers.
best thing for protecting it is 100% silicone grease. permatex dielectric grease was one source, dow 33 another but I'd make sure it did not go syn.
smear it on, let it set a day or so then wipe off. repeat occasionally.