I have used artist turpentine with chaulk pastels before and they work well....its a little thicker than water, makes more of a smoother slurry than straight water does.
Something else you can do with them, if you want to show like a fuel spill on an AFV or something, take Tamiya Clear, paint it on where you want to show the spill, and then lightly sprinkle on some pastel when the clear is still wet, it will mix with the clear, and once dry will look like a dried fuel spill