If your steering is stiff I would be looking at possible issues being mechanical or with the fluid or low fluid.
As far as using an additive, yeah it might work, but it's like putting a band-aid on a gaping gunshot wound. It doesn't fix
How old is the fluid in the steering system now?
One thing you might want to check out, the hose pictured below which goes from the power steering reservoir to the oil cooler pipe (metal looped pipe in front of the radiator) must have those two metal bands on it. Those bands hold a reducer in place to meter the flow of hydraulic fluid being drawn up into the pump. I know some people have removed those metering plugs and the wide open pipe/hose actually causes the steering to be quite a bit more difficult.
Check to see if you've got foam in the power steering reservoir at 2k to 3k RPM. Foam can also cause difficult steering.
Other issues might be the steering rack itself.