I finally had enough wrist pain with my Shen Hao, had to have one the price was soooo good, but it has had some idiosyncrasies. The focus is/was really stiff so I decided to fix it tonight. I took the stop plate off and removed the screws that hold the front standard tracks to the wood slide. I took a look at what might be binding, thinking it could be the wood on wood of the slide or the shaft, rail and knob. The gearing moved smoothly without the wood slide so the problem was in the friction of the slide. I used a utility blade alone to scrape off the finish of the tongue on the slide and not do anything to the groove. After getting all of the finish off it moved very smoothly and once assembled it is a joy to use.
If I ever had to sell it in original condition I could put the finish back on to restore it to the stiff difficult use, exactly how I received it.
And yes I had used it in the heat of the Southwest in Summer and in the Cold of Winter here at home. Temperature didn't have any effect on the friction. Now I will have to use the lock on the left for the first time. :o