Oiling out

It is necessary? In short the answer is no. As far as oiling out is concerned I will copy an excerpt from our “Varnishes” document (found in the Resources section) that may help to answer your question regarding sunken or matte areas of paint: Retouch Varnishes, “Sinking In,” and “Oiling Out” – Artists tend to apply retouch varnishes (or layers of oil medium) when they encounter problems with “sinking in” or areas of the paint that begin to take on a matte or under-saturated appearance (this is particularly common with darker colors). Sinking-in can be a result of a) using too much solvent to thin the paint b) using a ground that is too absorbent or unevenly absorbent c) if the paint film and/or ground layers are too thin and/or c) if not enough medium is present in the paint. Sinking in can also be caused by the painter using too much thinner, which will weaken the binder’s capacity to make a film, exposing the pigment to the air. As most varnishes available today are easily removable, it is not recommended to apply them between paint layers. While extremely thin layers of retouching varnish might not cause future problems, any paint applied atop retouching varnish will be more susceptible to delamination or damage caused during varnish removal (as is often done during conservation treatments). Application of successive coats of retouch varnish or interlayers of varnish and paint can also increase a paint film’s brittleness, again leading to potential adhesion problems and cracking.
Oiling out the surface of a painting can also be problematic as it can lead to problems with adhesion and long term solubility issues. Leaving an exposed layer of oil medium on a painting (i.e. regions of oiled out surface not covered by subsequent paint applications) will cause the surface to darken and/or yellow over time (in addition to becoming increasingly difficult to safely remove). Oiling out can be done carefully in between paint layers (or to cut the absorbency of the ground) during the painting process if artists consider the following recommendations listed below:
If your composition is complete and some areas still appear matte, locally apply varnish instead of oil to even out the overall sheen, wait until dry, then apply a final protective varnish over the entire surface.
To address problems with sinking-in during the painting process, try adding a touch of medium (heatbodied/thickened oil such as stand oil thinned in a solvent) to your paints or a problematic color or pigment (i.e. umbers are notorious for sinking in).
For oiling out during the painting process or for cutting the absorbency of a ground artists are recommended to 1) apply a thin layer of oil locally as needed or globally (consider using stand oil/thickened oil thinned in a solvent if your paint/ground layers are extremely absorbent) to matte/sunken-in areas 2) remove any excessive oil using a lint-free cloth and 3) wait until the surface is dry to the touch.
It is particularly important to avoid applying moderate to thick layers of retouching varnishes or layers of oil during the painting process as this could lead to potential delamination and/or cracking of the paint.
If you choose to thin your oil with solvents during oiling out take care if you are applying over fairly young oil paint as the solvents may begin to bite into the paint layers beneath.

Adhesion problems can occur with oiling out when the oil layer is to thick and/or the ground/lower paint layers are not sufficiently absorbent. Subsequent layers of paint will not be able to successfully adhere to the slick, oiled out surface and this can lead to delamination or other issues. One certainly can paint directly into the still wet surface….there is no problem with this, again so long as you avoid adding too much medium in general. It can be dangerously tempting to add too much oil medium at first. Try to avoid this as you will not only end up with a paint layer that will take forever to dry but also you risk creating areas of your composition that will be more susceptible to yellowing later on.