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.