As much as I may detest the use of "synthetic materials" for use as ingredients in my oil paintings, I greatly enjoy the use of synthetic resins as varnishes for my oil paintings.
It all has to do with compatibility. I don't feel that alkyd mediums are compatible with traditional oil paints. That makes them very questionable for use with traditional oil paint.
However, when it comes to the final varnish that is applied as the final, finishing coat to the surface of a finished oil painting, synthetic resins are to be favored, in my opinion, and it it BECAUSE of their incompatablity with the ingredients in the surface of the oil painting.
A good, final varnish for an oil painting should exhibit 3 important characteristics: 1) It should improve the appearance of the painting, by leveling out the high and low (glossy and dull) areas of the painting. 2) It should provide a degree of protection against dust, dirt, stains, and abrasions for the surface of the painting. 3) It MUST be easily removable, and without dissolving the surface of the painting, itself. This is for conservator purposes, as the painting ages, and the varnish becomes discolored over time, and must be removed.
The synthetic resin varnishes meet all three of these requirements. The synthetics are just a bit INCOMPATIBLE with the painted surface, and that makes them very desirable for use as a final varnish for oil paintings. The synthetics are not likely to bond, or cross-link, molecularly, with the natural ingredients (oils, solvents, resins) within the surface of the oil painting, because they are simply not composed of the same "stuff". Synthetic resin varnishes are easily removed with the weakest of all the solvents--Odorless Mineral Spirits, and without attacking the surface of the oil painting, itself. This makes synthetic varnishes the best choice for the protection of any oil painting. Most synthetic resin varnishes also exhibit at least some degree of Ultraviollet light protection, making them much better than most natural resin varnishes, in that respect.
My choice of synthetic varnishes is the following: For high gloss, I use GamVar Varnish, made by Gamblin. And, when I want more of a "satin sheen", I use Winsor & Newton Artist's Varnish, and I mix equal portions of their Gloss Varnish and their Matte Varnish, to create a "satin sheen" appearance. Each of these varnishes can be easily removed with Odorless Mineral Spirits, without fear of dissolving the painted surface beneath it.