An oil soaked stock probably took many years to get that way, and the oil has soaked very deeply into the wood. Solvent soaking and heat will get a lot of it out, but when I have put stocks aside for awhile after solvent soaking, but before refinishing, I've noticed that more oil usually comes to the surface to darken the wood again. And of course, your dirty solvent will have traces of oil in it unless you keep changing it out for fresh clean solvent.
On the flip side of this dilemma is the probable damage you are doing to the chemical structure of the wood by prolonged and repeated harsh organic solvent baths. I like to start with a solution of household ammonia to saponify the gun and skin oils, and to remove most of the embedded grime. Then I rinse thoroughly with water, and then use heating along with kitty litter or toilet paper wrap to absorb the oil that will come to the surface easily. Then proceed to denatured alcohol or stronger solvents such as acetone or lacquer thinner if necessary. Then do any dent removal, glue repairs, and refinishing before traces of remaining oil can come back to the surface to affect glue adhesion or affect final staining or finishing.
But to answer your question, no, I don't think it is possible to ever remove all traces of oil without damaging the structure of the wood.
