Hi Dana,
I've been musing on your problem for a while and was wondering if I might be able to lend a hand.
One of the tools conservators use (I hasten here to add that I'm a mere assistant curator, not a conservator, mind you) is the Teas fractional solubility chart. This is a handy diagram that (although it has flaws and limitations) gives a roughly accurate idea of the relationships between properties of different solvents. There's a pretty good Teas chart
here.
Anyway, looking at what you've used to far, it seems likely that your solvents have all tended towards the bottom right of the chart; while I'm no chemist by any stretch of the imagination, my understanding is that these solvents all work in the same way, and in this case that's primarily by London dispersion forces acting to force the molecules of the solute apart from each other. On the diagram you can see turpentine and benzene (which I'm using as a rough approximation for lighter fluid in this post).
I'm wondering if you might get better results from a solvent which has higher polarization (Dipole) or hydrogen bonding (I believe aka Keesom) interactions. Unfortunately, the solvents with high hydrogen bonding forces are also alcohols, probably not very good for the wood of the stock as they will displace water and then evaporate, leaving it excessively dry. As a suggestion, you could try to find some of this "Cellosolve" stuff mentioned on the chart.
For the metal parts, I'd try acetone as a starter if they can be detached from the stock. Again, acetone can be bad for wood (it's used as a water displacer during wood conservation, for instance - not a quality that recommends it unless you're planning to replace that water with something, since the acetone will evaporate and be lost quite rapidly) and I would suspect that its fellow ketones are no better, so detachment is highly recommended.
Hope this is useful,
Meredydd