91% isopropyl (rubbing) alcohol and oil of wintergreen in a 4:1 mixture does work, but if you soak parts too long they can swell. That happened to a few rubber distributor cam grease stoppers.
I just ran across this useful tip in RockAuto's January newsletter, which was just posted today, and thought it was worthy of sharing. I'll give it a try today on a few OEM nylon parts and post the results. The first part I'll try is an old yellowed nylon distributor terminal, I have a lot of...