Newbie here. Just purchased a 1985 5.0 mustang that has been sitting for a couple years. The car runs but the power steering is super stiff. The belt Is firm and not slipping and the reservoir is full of fluid which is filthy. Finally got it to break loose and the steering was smooth. Took out some of the old fluid out of the reservoir and replaced it with walmart brand of ps fluid. Now the steering is super stiff again. Is this caused by the wrong type of fluid or possible power steering...

Power Steering