Don't use soap - it's hydrophilic and will eventually make matters worse.
Very interesting point here. I saw the soap recommendation on YouTube and have been thinking about whether or not I like the idea.
Fortunately, I have no current tight stem issues; but I like to be prepared with tools, materials and solutions as much as possible.
Thinking out loud here (I’d appreciate any feedback): if soap generally is going to attract moisture, then theoretically, a tight stem issue would be eventually confounded if a pipe smoker used dry bar soap as a lubricant on the tenon. In other words, soap attracts moisture, which causes briar to swell. Further, the mortise & tenon are the most precise tolerance parts of the instrument, where moisture and temperature change are common and expected.
Has anyone experienced negative impact from using soap in this way?
I can’t think of any drawbacks from using graphite instead. If I had this problem to solve today, I’d reach for a pencil, but I would not be 100% confident with this solution. I have heard that graphite is mildly abrasive but I have never observed this myself, and I’d be interested to hear anyone else’s experience/observations.