I used hot vegetable oil on an already finished gun - a Tulle musket. The oil holds the heat longer and allows the heat to penetrate deeper into the wood. The oil itself does not go deeply into the stock. I believe I['d use water or steam on raw wood, rather than oil. Be patient and go slowly, reapplying the heat until the wood is heated through the wrist. You must have the stock clamped down securely and have a system for adding wedges or a twisting devise to the butt, a little at a time, and take the wood further than your need, as it will 'recover' and want to return to the warped state. It hopefully will recover only enough to make it perfectly straight again. Anyway, if it doesn't work the first or second attempt, try it again, until it does.
I applied clean shop rags to the wrist held there with nylon tie-straps, so that the heat would soak into the wood over a period of a couple of hours.
It was amazing how easily and far the wood could be forced to move without damage, when the heat was right.
