Why insist on an oil wash? If you are using acrylic paints, you may risk a reaction. Also, washes work best over a gloss coat so I'd suggest you apply an acrylic gloss coat before applying decals, then use your wash. If you do it on the flat coat, you will find that the excess wash will be difficult to remove in the areas that you don't want it to show. For acrylic wash, I use either acrylic paint or powdered pastels mixed with water and a drop or two of dis soap.