I don't think there is a magic material that can be added to dry cracked hoods to make them soft like new again. But, here is how I rejuventated some original 1985 Campagnolo shield-logo hoods for a TREK 510 two days ago :
1. Clean the hoods. If you can use soap and water, simple green, or whatnot, more power to you. For my terminally dirty (but supple) hoods, I used acetone. Be careful as this solvent will remove a thin layer of rubber from the hoods, and make them very sticky. Do not wipe the shield logos more than once, or you may accidentally remove the logos.
2. Wax the hoods twice with a high-quality polymer wax, such as NuFinish or Raindance (the type of wax that lasts a year). This will restore the slick finish, and helps to seal the hoods from damaging ozone, which will eventually destroy the natural gum rubber in the hoods.
I have used the same procedure on Carlton hoods. The logos are just as easily dissolved by acetone on Carlton hoods as on Campagnolo. The carlton hoods get very dull after the acetone treatment, but the wax helps to shine them up again. The benefit of using acetone is that it really does remove a thin film of plastic, which brightens up yellowed hoods like a good mouth-bleaching set might do to a smoker's teeth ...
- Don (who doesn't smoke) Gillies
San Diego, CA, USA