I have started to experiment with oil based paint washes. I started by applying a darker wash to the cylinders of an air cooled radial engine, to give the fins some depth. I like the effect, but the mineral spirits I used to thin the oil paint, softened the base coat of paint, and it will come off just handling the engine. Is there anything I can use to thin the oil paint that will not attack the enamel base paint?

Was the base coat enamel or acrylic? If it was enamel, it will react with almost any thinner. you can add a coat of Future or other acrylic gloss coat to protect it from an oil wash.

I use a thinner purchased from an art supply store. Rembrandt low odor, which gently thins the oils for a nice, even wash but doesn't attack the paint underneath. Anything from the hardware store is too harsh.

I believe you are right. I was using enamel, and did not put any kind of clear coat down before I applied the oil wash. Before I paint the rest of the model, I will spray a clear gloss coat on before the oil wash.
Thanks!