You found it alright, that little black knob. And the M/B suggested products only list M/B Fluid. My '87 W124030 has 170,600 miles and has had no steering gear or pump problems. A little trick I use is as I change my oil from the Top Side with my El Cheapo Oil pump, I remove the P/S pump oil filter and using my pump I suck the resevior empty and refill it with Valvoline Synthetic P/S fluid. I do this ever 3500-4000 miles. I also do something similar to the transmission. Overkill? Oils Cheap, Pumps and Gears ain't. As for noise, the power steering pump at end lock can produce up to 1000psi, noise will be heard. This is for the cirulating nut steering gear, not Rack & pinion which is another topic for discussion.
Happy Trails Beep Beep from Houston.
AJ likes Vavaleen