This subject has been broached before but I am happy to report success in revitalizing old rubber.
I have been using a mixture of 30% wintergreen oil (methyl salicylate) and 70% xylene to restore old rubber parts and it works beautifully. This is the same ingredients and proportions for the commercial product Rubber Renue according to the MSDS sheets.
Wintergreen oil is analgesic and liniment for aching muscles and found in most drugstores, pharmacies, health food stores and co-ops.
I gently heated rock hard honda CB carb boots with a heat gun for 30 seconds and dropped them into mason jars with my solution for 12 hours at room temperature. They come soft, pliable and looking good. They have been out of solution for 48 hours now and are still soft and pliable. I soaked smaller rubber parts such as side panel grommets and there extremely soft and may need to soak for less time
Critics are going to argue 'why not buy new ones?' Well, I am restoring four CB's at the moment and if I can reuse perfectly good parts, why throw them and my money away?
Give it a try and post your results.