I know I'm a little late to this party, but I thought I'd put forward some information that might help.
Whilst not exactly tested under laboratory conditions, the testing this fella carries out is pretty comprehensive and he uses a fairly convincing methodology. It converted me to the 3:1 mix of isopropyl alcohol and Wintergreen, which is by far the most effective method I've used for resurrecting rubber:
https://www.youtube.com/watch?v=EnGsllKvheQ (https://www.youtube.com/watch?v=EnGsllKvheQ)
Sorry, can't work out how to embed. Anyways, I've since found that there are some other things you need to be aware of if you go this route:
- the mix works best with rubber that was once reasonably malleable, like connecting tubes and the air box intake tubes on the CB750. Somewhat unsurprisingly, the effects are less noticeable on hard rubber parts like cush drive pieces. I've no data on this, but I imagine the rubber compound(s) would be the major determining factor in the swelling.
- as alluded to in the video, the thinner the rubber, the more likely it is to swell. The effects of the swelling can be quite alarming, although I've found that most parts will return to their original size within a few hours. The exception to this is the lips on dust/grease seals; they swell and stay that way.
- the amount of time parts are submerged in solution can also effect the amount of swelling. Depending on what parts you're treating, you'll may need to keep an eye on them and not just leave them submerged indefinitely. But again, stuff like stoppers or cush drive bits don't seem to deform regardless of how long they're in.
- high concentration Wintergreen can also be found in equine supplies under the name horse liniment. Whether that makes it any cheaper, I'm not sure.
- find some large jam jars or pick up some cheap mason jars next time you're in Ikea in which to store your mix, and where possible get them large enough to fit the rubber parts in (one at a time if necessary). This stuff doesn't seem to go off and remains viable so long as the alcohol can't evaporate. So while it may seem expensive, you can use the same container over and over.
- it works astonishingly well on hardened HT leads. Get a piece of plastic pipe (22mm is a good bet), seal one end, fill it with the mixture and gently force the tired old leads attached to coils into the pipe (if they're anything like the age-frozen leads I've been dealing with, this bit may take some patience). I had to leave mine for several days, but they eventually became more or less as flexible as they would have been when new. In all seriousness, this is the one thing I wish I'd known several years ago because it would have saved me from replacing coils where the leads had hardened.
- it doesn't seem to have any noticeable impact on bare metal. I've left bare metal parts that have rubber pieces in/on them in solution and the metal doesn't seem to change in any way.
- whatever you do, don't keep powder-coated parts in close proximity to the solution for any length of time. I soaked a powder-coated engine mount that housed a hardened rubber bush to see if the bush softened, and the powder-coat literally fell off the metal. I've not seen a coating lift off a surface it had been applied to like that before, even with paint stripper. Whether it does the same with pained surfaces, I can't say because after the powder-coat exercise I haven't repeated the test (for fairly obvious reasons).
Hope that helps. If I remember anything else, I'll edit this post and add it here.