When I soften thing up , I use a ziplock freezer bag.
I usually use the Green stuff, soilant green? That's not it, but it has green in the name ,you can find it anywhere.
I leave a little in the bottom of the bag , toss rubber items in the bag, shift them around anytime I remember to , and the rubber parts have always been soft...and clean.
I think that Green stuff smells like it has wintergreen in it. smells nice.
Oh, just remembered....Simple Green. That's it.