I believe there are products, as you describe, that will help prolong the life of rubber if used periodically from the start. However, despite claims, I've never found anything that will actually rejuvenate old rubber once it has become deteriorated. Best solution I believe, as mentioned, is get a new one.

Now if you want to give a try, I participate in a vintage motorcycle forum and this comes up quite regularly as you might imagine. Some 'claim' success using wintergreen oil. Good luck with it, let us know how it goes.