On your 190, I'd try to get the MB stuff. You gotta get into rather oldish MB's before I'd use ATF. Go with the owners manual as a guide, if MB says to use ATF, then that's what you use.
So try to get the MB power steering oil, and what I'd recommend if you really CAN'T get it easily enough is to find any type of product that says on the container that it is "Power Steering Fluid".
PS If I were stranded somewhere and say I had a major power steering leak, and I fixed it and now I NEEDED some fluid in order to avoid damaging the steering system, and all that was available to me was ATF, then I WOULD use it. It can do the job, but if I am somewhere where I can pick and choose I'd get the MB fluid if possible.