Surely it is not shellac, though shellac is a resin, and that's what you get (a resin) when you modify an oil. Boiled linseed oil used to be modified by heating it (hence the '"boiled"). But now it is treated with the "japan driers" which are toxic metal compounds like cadmium and a few other tasty ones. These chemicals speed up the natural drying process in which "cross-links" occur between certain molecules in the oil. This is what happens when an oil "dries". Certain oils such as tung oil will dry naturally, while others will not. Linseed oil is somewhere in the middle.
So really, try cutting polyurethane with mineral spirits, 1 to 1 or more, making it thinner. Apply with a rag and wipe off the excess. This is a useful finish for many things. Watco oil is a similar type of finish (called a "danish oil"). Also wipe-on poly is basically poly that has been cut with mineral spirits. Mixing in a little boiled linseed oil would probably just slow down the drying time.
