sooooo... teach an old dog a new trick. I recently ran into a formula for softening age hardened rubber... couple ounces of wintergreen oil in a pint or so of xylene. I just did some intake boots & they went from rock hard to like new flex in 24 hours. I am astounded at how well it worked. the wintergreen is supposed to have a natural plastisizer in it called meythyl salicylate. the test bits did not swell up noticeably and are still pliable after a couple days sitting on the bench. I put another set in mix yesterday & they show improvement, but not as dramatic as the first set. I intend to leave them in until tomorrow & see if they continue to improve. It's the same mix though, so maybe some fresh materials would speed things up. I got the oil from a local health food store, another source is homeopathic medicine. I hear that sports medicine uses it too.