P/S fluid advice

[frosty]

Car is a 1990 190E 2.6

Lately I've noticed a difficulty in turning the steering wheel in tight spots, like when parallel parking. No groans or other weird sounds coming from steering wheel pump. So before I get to it tomorrow, could someone tell me what fluid I add to a pump resevoir containing unknown fluid? I don't know if ATF or P/S fluid is in there or not.

Bonus question. Is it bad driving with low or near-empty fluid in P/S system? I think I've been driving for quite some times with low P/S fluid. I know my resevoir or pump seeps fluid onto the sides of the resevoir. I never bothered to add any fluid as the steering was never adversely affected or became extremely difficult to steer.

[Ferdman]

Is this a trick question? Use power steering fluid in the power steering unit. Yes, it's a poor practice to drive your car when the power steering fluid reservoir is nearly empty. That can damage the power steering pump.

[mbshop]

yes, p/s fluid is required as opposed to atf. no noises then it
should be ok.
wondering if maybe some ball joints or steering parts are rusted
enough to cause this problem?

george