The MB maintenance manual prescribes "hydraulic oil" for the throttle linkage for USA version vehicles from model year 1974. (This was printed in 1979)
It goes on to say that only the following grades of hydraulic oil may be used:
BP-Aero Hydraulik 1
Castrol DB Hydraulik Fluid
Esso Univers J-43
Mobil Aero HFA
Shell Aero Fluid 4
Frankly, I would question even how many dealerships follow these instructions.
Anyhow it would appear that you should use a somewhat more substantial lubricant than WD-40, as great as it is.
Have a good one.