What is the question? What have you checked so far? I can't find any reference to a 180SX model so I can't look up parts. You didn't list the engine size. Are you saying the oil change is related to the hard steering? If so, check for oil spilled on the serpentine drive belt. If your car uses the small square "ZF" style power steering pump, they develop weak output at low speeds. Replacement is the only cure for that. The clue is the power assist comes back when you raise engine speed and the lack of assist is always equal in both directions.
If the loss of assist is worse in one direction, that is caused by worn seals in the rack and pinion assembly. If your car has the electric power steering setup designed to easily retrofit to an electric vehicle, first check for proper generator output. If that is okay, suspect the steering gear assembly. Those are REAL expensive so hope you don't have one of those.