David Farber writes:

I just bought a bottle of Rubber Renue. I haven't tried it yet though.
If I find any wear on rubber parts, I like to replace them. I really
have never found a chemical that can rejuvenate a belt or an idler for
any length of time.

Nothing truly renews degraded rubber, which is due to chemicals like ozone
reacting with the polymer, due to the shortening of the polymer chains with
age and other factors. Now you can coat a rubber thing with stuff, and the
stuff will change the surface characteristic slighty and for a while, but
you haven't done a thing to the bulk properties.

I used to put a thin coat of gel toothpaste on tired rubber rollers for dot
matrix printers and fax machines, back in the day. Worked well, if
briefly.