Roofing tar will work fine. For a long time, I would dig it up off the tar and chip road I lived on before they put hot mix on it. Then, I went to roofing tar. It will enhance the open end of the grain of the wood that soaks up the liquid more than the grain that isn't cut across the grain. It really enhances fancy maple grain. It looks like a mess, but it cleans up well with the steel wool and it being wet gives you an idea of what color you will have when the final finish is applied. In some cases, I have used 320 sand paper in conjunction with the steel wool to speed the process up a bit.
Roger