To add my tuppence/knowledge, if it helps at all;
When the fluid is low due to age/leaks/whatever reason, not only does the steering get affected slightly, but the pump itself gets damaged - this is because the power steering fluid is actually lubricating the pump as well as working the steering.
When fluid runs low, the pump *will* get damaged to some extent - so if you know you have low liquid, STOP driving immediately, and fix/top-up. I know a guy who thought he would just have a difficult to steer car whilst he got home, and drove 5 miles or so and completely knackered the pump.
One way to check for power steering pump damage is to start the car, and turn the wheel to full lock and hold it there. If you get the groan, it's damaged. *However*, providing you fix the leak/top-up, you can keep a partially damaged pump going for years/ever. As long as you can live with the sound!
If I had an old Ford with the noise, I'd just ignore it, but not sure I would be able to with a porker
