What we use and are very pleased is for the really tough ones try GM General Purpose Penetrant and Heat Valve Lubricant Part 1#1052627 I also have found that the MOPAR Rust Penetrant 04318039AB works equally as well and is a little cheaper. I buy these by the case from the local dealer. I also use heat quite offten. For just the ones that are gummy, lightly rusted I use Sea-Foam Deep Creep. Experiment and see what work best for you.
J
