I still have a pretty hefty pile of gilsonite that Bill gave me years ago. Thus far, it is the only 'asphaltum' that I've worked into an oil that acts transparent as opposed to a pigment. Roofing tar, whatever it is, always acts like a pigment - it clogs up and enhances grain lines, which is not preferable (to my mind) on a gunstock. Gilsonite, when heated into boiling oil, does not do this. It acts more like a transparent dye, and the material itself initially is the same hard consistency of a tree resin.
Coal tar seems to create the same pigmentation issues. Muddies up the grain.
Pine tar as sold in Agways and other comparable stores will work well insofar as darkening oil without excessive pigmentation, but it's pine resin based without the enhanced purification that hard conifer resins go through and there is no way to render it as a hard finish. It's sticky and gummy.