I soak nearly every pen with a "natural finish", ie sanding and polishing. Soaking can occurs any time after the last glueing (glue does not hold well on oiled wood), often at the very end after assembly: it does not hurt metal nor plastic threads. In my opinion:
- It keeps the wood fibres down, stuck to the wood, even under dashes of water, which are often visible on unoiled woods.
- It is the only finish that gets better with time, just have a look to a one century old tool-handle. It can be worked up to a very good shine on hardwoods.
- It also adds a little colour to some woods like thuya, but I wonder if a light acid wouldn't do the same. It warms up any wood because of its very light yellow colour.
For the best results, I need 3 dilutions: the first (often the only one) for soaking, diluted with 1/2 turpentine, for a deep impregnation. The second with 1/4 turpentine is supposed to full the pores (I nearly always skip this one for reason of laziness), the third with pure oil, applied with a brush for a shiny surface. Long drying time in between: at least one week in summer, more in winter. The longer the better. If I am in a hurry, I just soak it in the 50/50, wipe off the excess and that's it.
I use non boiled oil only because I can get it for cheap at any Leroy-Merlin. Boiled oil is hard to find and very expensive, the only advantage is that it dries a bit quicker. I used to add some accelerator, but this poison stinks and I don't use it any more. Turpentine is real turpentine, not a petrol spirit dilution.
On Thuya, the penetration doesn't work so well you are right, but it works. Sometimes resin comes up to the surface and changes the touch. But it is better than nothing I think.