It'll probably break. WD-40 probably won't help. I'd think about shutting the WH off, letting it cool, and then just try to unscrew the whole valve and replace with a decent one. While you have it off, if it doesn't drain (may not much sort of like a straw with your finger over the top (assuming you close the inlet supply), you could take something and poke through the crud to help loosen it and let it drain. Depending on how old it is, it might either help, or speed its life or death.