Factory oil was SAE 5 or 10 or 20 motor oil pending ambient temps. I use a general purpose hydraulic oil. Some people use 10-30 motor oil. System doesn't need a special oil just low viscosity suitable for hydraulics. Torque capacity is ~2 gals.
Pic is early 50's model after the return filter was developed. You may not have the filter. Full level is to the lip of the plug opening.
There is a breather mounted on the top forward of the steering stand, might want to check it clear.