I loaned him my can of KRoil. He soaked the plugs for 3 days and then screwed them right out. I gave him the can for future customers. One note of caution, though, this stuff works almost too well. Once you get the part out, clean up the KRoil and don’t leave it in. Otherwise, your parts may work loose and fall off.
https://www.amazon.com/Kano-Kroil-Penetrating-liquid-KROIL/dp/B000F09CF4

