I have used no less than 8 different clay products and Z18 is my favorite. It is stiffer and harder to knead than the rest but I don't use clay that way.
I find it MUCH safer to cut the clay in small slices, shape it to silver dollar size pieces, use one side until dirty, flip use the other side and then toss it. It is very economical and you will never risk the temptation of kneading debris into a clean bar and potentially damaging your paint in the future.
I gave several demo's on claying at my Tips and Tricks event and folks were amazed at how easy it was to use even for first timers. Z18 was used and diluted Z7 was the lube. It never left residue, stuck or scratched.
Thumbs up to Z18.