Steve, I am 99% confident the quality of your oil paints isn't the issue. Even cheap oil paints these days are made using a refined linseed/safflower oil, and are of a relatively decent quality. The difference in cheaper oil paints will be in the pigment, not the binder. Cheaper paints rely on more pigments to achieve the same colors, and often use fillers to get the consistency they want. More expensive paints will usually rely on a single pigment for a color, and they won't contain fillers. I have been traditionally painting with oils for a few years now, and spend a heap of time researching paints.
As others have mentioned, it sounds like you need to isolate your base layer with a clear coat, or use a less harsh solvent to thin your paints. You could also consider using plain oil to thin your paints down, but the viscosity would likely be far too high to effectively work as a wash.