PB Blaster is the only penetrating oil I use. Wire brush around the bolt so any external chunks of rust is out of the way. Then soak it, heat the manifold, and rap on the end of the bolt with a hammer to give it a jar. Then heat it up some more...and more. If the bolt doesn't want to come out by then, you're not getting the manifold hot enough. Try propping the manifold up with the torch pointing at it while it's standing up. Then take a 5-10 minute nap while it cooks.