Well, there is synthetic and conventional, so I guess there are two types of steering fluid. Since it is a hydraulic system, you could also use jack oil or brake fluid in there as well. Although they may be detrimental to its longevity and performance.
I have used atf for brake fluid before to get a broken 4x4 off a mountain and into civilization. It attacked all the rubber in the system and it required new master and wheel cylinders sometime later to restore adequate safety.