The sell by date is more of a marketing date. It frequently doesn't have much to do with when the food actually goes bad. It merely indicates when the company would like you to throw away the old stuff and buy more.
That said, my mother used to be on a prep list that has since died away. One of those people wrote to a shortening company (might have been Crisco, but I don't know) asking about if shortening went bad. A representitive wrote back saying that the shortening does not support life. They meant microbial life, mold, etc. But the point is that nothing bad will grow in the shortening. The taste may go off, but it takes a very long time to go bad.