Tough Steering (when cold)

Two days ago I flushed my power steering fluid since I found 3 quarts of generic brand Mercon/Dexron III ATF that is of no more use to me since my dad sold our old Taurus a few years ago. I figured I'd use it to flush my power steering as it's been almost exactly 30,000 miles since I had it done at the dealer. It was pretty dark and smelled old compared to the new stuff.

I used the "turkey baster" method to suck out the old fluid from the reservoir, topped it off with new fluid, started the car and turned the wheel lock-to-lock several times. I repeated the process four times until it was bright pink in the reservoir after turning from side to side. And the fill is exactly in the halfway fill markings.

Yesterday I drove it around town and on the highway and noticed no problems. But last night when it was about 5*F, my steering felt rather tough to turn. It did not feel like the power steering was inop, just not as easy as normal.

My question is could this be the fluid not performing up to par or is it more likely some seals/joints on the steering shaft? I have a spare bottle of Mobil 1 Mercon V if necessary to refill the system with that.

BTW, there's no unusual noises or anything coming from the power steering.