Many have no idea where to find the power steering reservoir on the M120. It is genuinely hidden in plain sight! Also questions that pops up are which fluid do you use for the Mercedes W140? I will answer both.
Open the hood and look down on the engine. Then take your finger and point to the first star emblem you can see.
The star is part of a plastic lid. Remove this, it is held in front by two tabs and then you slide it towards you.
Now point at the container with the cap on it. Congratulations you have found the power steering reservoir.
Open the cap and measure the . I am pretty sure the level will be low. This has to be the most neglected fluid on cars. This is the main reason the steering box gets sloppy, due to the fluid running low, wrong type of fluid and just pure neglect.
The fluid you should use is Febi Lenkgetriebeöl 08972 or an oil with MB 263.3 approval. Alternatively you can get it directly from Mercedes. Do not use ATF for the love of God. AFT works, but will wear the steering box prematurely.
Cheers, Robs out!