Yes, there are experimental questions, but you will have no idea which ones they are. The best thing, especially knowing that some questions won't contribute to your score, is not to stress too much over any one particular questions. Just give every question your best effort, even if it ends up being a guess. There's no sense wasting time on a question you can't figure out, experimental or not, so don't be afraid to make some guesses. If you can narrow down the choices to 2 or 3, all the better.
But don't go guessing willy nilly every time you come to a tough question - if you need more time to figure it out, mark it and you can come back to it if you have extra time after you've answered all the questions.
Oh, and 20% sounds way too high. I heard there were about 10% experimental questions.