There are, I think, two kinds of clay, one is more plasticy than the other. My Clay Magic says, no, there is 16 or 17 different kinds. I don't buy it.
But there are two grades: fine and medium/hd. For a dark car, never ever use a more abrasive grade unless you have to because of the degree of contaminants.
Buy an OTC kit (like Mothers or Megs) or by the QD and clay separately, which are usually fine grade. I find I can dilute QD by about half and still have a nice lube. Soap and water works to, but isn't quite as nice to work with. The soap also breaks the clay down a tiny bit faster, so if yo0u have a cleanish car, it means turning the clay in (or kneading it) more than you need to.