1988 560SEL HYDRAULIK OEL reservoir

Friend of mine has a 1988 560SEL body is W126.  I fix my own cars -
not Mercedes though - and therefore he asked me to take a look at a
low fluid level at a reservoir towards the front of the engine
compartment drive side.  The lid states 'hydraulik oel', which really
just means hydraulic oil.  I am suspecting that this is the power
steering fluid.  Could anybody confirm?
He does not have the owners manual which should have a sketch of the
engine compartment labeling what it is.  Online you don't find one
either that can be downloaded.  Maybe somebody could scan the page
from the manual and send it.

Thanks in advance for your help.
Axel