All about sinking in and oiling out

​If paint is lifting when oiling out, the initial layer is probably not dry enough yet. You may want to try a different medium, maybe alkyd-based. For a very long time, artists have employed mediums that render paint faster drying and, when touch-dry, less vulnerable to staining and re-wetting. The de Mayerne manuscript describes (vaguely) the “Amber Oil of Venice” that was used in the Gentileschi studio for flesh and white passages, to facilitate prompt overpainting and reduce staining of light colors. There are lots of factory-prepared mediums that deliver these benefits today. A medium that imparts gloss will also reduce sinking of colors.

If the paint film contains significant amounts of wax, I would not advise oiling out or any heavy manipulation. Wax tends to dull a paint layer so if you are having difficulty achieving deep darks, wax may not be the best choice.

Some of these are well answered in our resources section about oiling out. I am sensing that you are restating these questions because you do not really like the answers we have provided 😉 I will briefly address each of your questions.
The fat over lean “rule” is an over simplification. The important notion is more rigid and brittle under less rigid and more flexible. In this instance, this is not the important factor. If you rub walnut oil into the acrylic dispersion ground and paint into it before it has dried, the oil becomes incorporated into the paint and is not a discrete layer any more. We really advise to not apply oil as a coating that is allowed to dry before paint is applied. The reasons for this are enumerated in our resource section. A properly dried paint film should be resilient to the application of additional layers of paint containing solvents as long as the solvent choice is not too “hot” or slow evaporating. This is one of my issues with spike oil which evaporate so slowly that is can dissolve paint films that have set for a reasonable amount of time.
Bodied oils tend to provide a slightly sticky surface after the solvent has evaporated. This may or may not facilitate your intended effects. In addition, stand oil is less yellowing that unprocessed oil. There is nothing wrong with using walnut oil, as long as you are going to paint into that oil and cover it completely with layers of pigmented paint. I sense this is not what you want to hear.
Unfortunately, it is a bad idea to leave dried oil exposed, even over dark paint layers. It will yellow, but also become turbid over time. These areas will eventually be seen as slightly brownish-green. No, it will not be as visually disturbing as exposed oil on light colors or white, but it will still become visually apparent.
There are too many factors here to give an exact timeframe for yellowing. Again, I am sensing that you want to hear that if you have not seen this in X amount of time, there is not a problem. I cannot offer that assurance.
Unless the effect created by additions of wax are essential to your aesthetic aims, I caution against incorporating it INTO oil paint. There is not a real problem with this but if large amounts are added, the work should be treated as something different than a traditional oil painting and certainly information about the media and wax should be noted and retained on or with the painting. You should also seriously deliberate whether to varnish such a work. Wax will remain eternally soluble in mild organic solvents making it difficult to remove a discolored varnish if a substantial amount of wax has been added.
If you oil out into a couch of wax you are likely going to slightly dissolve the wax into the oil. This would be preferably to having a discrete wax layer between layers of oil paint. Wax interlayers could be “undercut” during varnish removal, which could scale away the uppermost oil layer. This is the worst-case scenario but it is certainly possible with wax interlayers. Again, please record you working materials and layering on the painting if you are going to use this technique.  You could also make a note on your painting that you would prefer that your paint not be cleaned in the future due to your medium or varnish decisions. As always, it is better to provide future conservators with more rather than less information.
I hope this was of some help.

There are a couple of different components here. One of the main concern with oiling out is that any portions that are left exposed WILL yellow over time and become unsightly. We see this frequently on paintings that enter our conservation lab to be cleaned. These regions remain irretrievably darker and yellower than the sections of the surface that received subsequent layers of pigmented paint.
The nice thing about painting into wet oil is that it becomes incorporated into the paint and is not a discrete fatty layer between more rigid layers of paint. If this layer if substantial, it will violate the general principle of keeping the more rigid layers below the less rigid layers, This would be even more problematic for canvas paintings as generally flex far more than panel paintings.
However, one can successfully apply a minimal layer of oil/medium to cut the absorbency if one is rigorous about only adding just enough to achieve the desired effect.  I know that some knowledgably commentators state that this dried layer can be left uncovered BUT my experience in conserving paintings leads me to caution against this practice.
Finally, I do think that your proposal to lessen the absorbency of a Clayboard panel is doable. I have done this to the same product when my project required a less absorbent ground that the untreated product. I will contact Amperstand to get their take on this as they have done a lot of R & D and testing of their products.

A VERY MINIMAL amount of alkyd medium rubbed on very thin using a pad of lint-free cotton fabric. This was allowed to completely dry before working on it. It was later completely covered with paint.