The only advantage I can see to painting on paper in oils is the cheapness of the support compared to canvas. The practice was to execute quick studies in the field on relatively inexpensive supports, the purpose of which sketches was to serve as reference material to aid the artist in the execution of a more important picture, which would be painted on a better quality support. The sketches and studies were not important to the painter once their purpose had been fulfilled and the big picture was finished, so archivalness was not a concern. The surviving examples of these sketches survive in spite of the painters' relative lack of concern for their longevity, because their reputations were high enough to give monetary value to anything they had done, and the best conservators were engaged to preserve the sketches after the artists who created them had died. Do not overlook the very strong possibility that many of these sketches on paper likely did NOT survive the centuries.
Linseed oil, the binder in oil paints, is acidic in its pH, thus can be expected to initiate and accelerate the rotting process if allowed to penetrate the fibers of paper. The purpose of the shellac you mentioned was to seal the absorbency of the paper in order to prevent it from drawing oil out of the paint, which would extend the life of the study to some degree, but more importantly to the painter, it would allow the paint to behave in the manner to which he was accustomed, and not go matte from the paper sucking the oil out of it.
Another point to consider would be that paper, in the days those older studies were executed, was made from cloth scraps rather than wood pulp, and was usually sized with something or other in order to keep ink from spreading.
For an inexpensive support for plein-air sketching today, there are better things available, including hardboard (Masonite, Presdwood, etc.), which can be primed with the primer of your choice, and Fredrix has recently come out with archival canvasboards in a variety of types at reasonable prices.
Of course I don't know whether you're a student with no money, or a professional artist, as I haven't seen your work, so bear that in mind as you read my comments. If you are a student, and not very good yet, it might not be of any importance whether what you are doing now survives over the centuries. If that is where you're at right now, you will probably be so much better in ten years that you won't want anyone to see your student work anyway, and the quicker it disintegrates, the better it will be for your ultimate reputation. Some painters have intentionally destroyed their early work, or at least the worst of it, selectively editing to avoid having the worst things they'd ever done ending up on exhibit somewhere with their name attached to it after they're dead and unable to have a say in the matter, things they had never intended the world to see or consider a suitable representation of their abilities. Many painters have expressed a desire to have only the best things they'd done survive. So these are all things to think about, but if you're pretty good and people are buying your pictures, professional ethics should compel you to use archival materials, and not scrimp on supports for your paintings. Your collectors and posterity will appreciate your conscientiousness in that regard.
Good luck with it.
Virgil Elliott
