The SLS fluid, at least for the older cars, is actually an obsolete aircraft hydraulic fluid. The owner's manual for my 1981 300TD has a list of acceptable fluids that includes Aeroshell Fluid 4. Your local FBO will know it as MIL-H-5606A and have it in gallon cans.
This is not the same stuff you want to put in your power steering reservoir. For one thing it's rather thin compared to Dexron III.
I have heard that the seals and diaphragms in the SLS system are incompatible with ATF and will soften and fail if you use it. I did briefly have "tractor fluid" in my system and it stopped a couple minor leaks, which I suppose suggests seal swelling that could turn into degradation over time.