I have also found that Kerosene works in some cases.
fluid thinned with a little acetone or MEK might work--but mos t of these treatments have to be done multiple times over several hours to get any degree of penetration.
No doubt you have used all of the proprietary ones--the WD40 and 3in1 oil. Some people swear by eucalyptus oil--but not having tried it I can not say. It might be worth a try.