Thanks, Sharon. If I do break down and use an OMS, Gamsol would likely be my choice. I just worry about long-term exposure to it in my studio, which isn't the best-ventilated studio on earth. Some say that odorless spirits are more worrisome precisely because you don't smell them, and thus don't sense when they're invading your head. I dunno.
I received my bottle of Bio-Thinner from the Art Treehouse and gave it a try. It thinned the paint nicely, like OMS. I used it to paint a ground over a panel, and I also used it to wash brushes. It made for a nice thin wash, and it was effective as a brush-cleaner too. I don't love the orange-citrus smell, though; it reminds me of Turpenoid Natural, which always has struck me as somewhat unnatural. I worry that inhaling the stuff may be no better than inhaling an OMS like Gamsol.
For cleaning brushes, walnut oil has long worked fine for me, and I'm inclined to stick with that. But walnut oil won't do for an initial wash, so I'm going to continue to experiment with this Bio-Thinner product.
Incidentally, soy-based solvents apparently live a bit of soy residue behind, unlike OMS, which evaporate entirely I suppose the worry is that the soy solvent won't create as strong a bond in the long run? I have no idea whether this is anything to be concerned about.