Generally that means you have lost enough VOC and volatile oils out of the rubber to be pretty much bad. Thicker cross-section parts like forward intake manifolds are not the same as the thinwall rubber boots at the back are, and the thin parts getting treated to be more flexible generally tear much easier.
You know of course that exposing the parts to like solvents will swell them out of shape, you could try a lesser swelling agent like dumping the parts into oil for a while to partially imitate the oil lost from the rubber, just an idea with no guarantee of working. I've done it on the side cover grommets before, to let the covers come out without cracking them.