Steering stiff at 30 degrees Fahrenheit

Unfortunately it is normal as the fluid needs to warm up to make the steering act like it does in warmer temperatures. About the only thing that can be done to help would be to flush the system out and replace the fluid with a synthetic version. The service specifications show it takes a liter of ATF Dexron 2 or 3 with the Dexron 3 spec being a synthetic.

Not really any harm, it just takes more force to move it which means things like the steering shaft bearings and joints get a bit more wear and the seals in the steering see a tad bit more wear. The pressure inside the unit goes up a bit as well because the thicker fluid is harder to pump and it doesn't flow through the passages as easily. However it should only be stiff for a short time, as the fluid is pumped through the system it will warm up and become easier to turn. As long as you used the correct fluid in it you shouldn't have much of an issue. I like to run synthetics in everything I can just to make the cold easier to deal with.

Shouldn't be anything in there to filter, plus it would be another restriction.

On that truck you are lucky, it has an easy to remove return line on the side of the reservoir. You start by removing as much fluid as you can from the reservoir, then remove that line and cap the fitting on the reservoir. Put the open end into an empty container large enough to hold a couple gallons of fluid. Now fill the reservoir with the new fluid and have someone start the engine while you add fluid, have them turn the wheel full left and full right and run a couple liters through it. Shut the engine off, Remove the plug from the return fitting and put the return line back on. Then top off the fluid, start the engine and turn the steering wheel lock to lock. Shut it off and top up the fluid to the cold level. Let it set overnight or so and check the level again. Done. Two important things, keep the fluid going into the pump as close to the amount coming out and don't let it run dry. You don't want a bunch of air in the system, it can make the pump noisy until it purges out of the system.

It pumps the fluid through the entire system, unless you use a powered flush system it's about the only way to get all the fluid changed.

Unless it leaks or gets contaminated it should go 100K easy in normal driving. I don't even find a service interval for it on that truck. What I normally do when that happens is look at the fluid. Is it still the same color and feel as new fluid? Is it black and gritty or has a milky look like it has moisture? If it falls under those, flush it out with new. If it's still close to new then it's not worth the aggravation unless there is a problem.