There is a product that worked on my airbox rubbers... Rubber Renue
Before they were unusable - hard as a rock.
I hit them with a heat gun and that softened them a bit, but as soon as they cooled off - hard as a rock again.
Then I found some Rubber at a local electrical supply house and got a few bottles (they were about $4 each)
poured it in a coffee can, heated the rubbers with a heat gun, and dropped them in and put a lid on it - I could only do 2 at a time.
Left them in about 24 hrs. They swelled up quite a bit larger than original size, but they were very soft and very pliable.
After another day or so, they went back down to the original size...not quite as soft, but DEFINITELY better/softer than
they were before I started.