I saw in a tutorial video that, in animation, the lip sych could be done well enough to read the lips. I'm wondering if this is true/common or not. Can you read lips on Pixar or Dreamworks movies and keep up with what is being said?

What about older hand-drawn 2D animation?

There are some letters and combinations that are easy for anyone to read, maybe you can try to watch a movie (like Pixar's movies) and try to see if the lip-sync is really that accurate

and also certain sounds need certain positions made with your mouth, you just have to watch yourself talking in front of the mirror and observe it (also, depends of the language you're talking)