Don't make such a big deal of it
OEM or not, Follow the hoses from Reservoir to pump to steering control to cylinder. Something is not working, and you need to know what.
If that is the reservoir you have pictured, Fill it with hydraulic fluid. Leave the cap off and run the tractor to see if the steering is restored.
If it is, You were low on fluid, and you should pull some of the fluid out of the full reservoir (about 3/4 full as mentioned would satisfy me) 1/4 poly tubing and a suction gun if needed.
If the steering is no better, your troubles are elsewhere.
There are no oil based systems on a tractor that can be hurt by using hydraulic (UTTO) fluid to check things out