New plaster is by nature, porous, as such the oil will have soaked into it. Adding other liquids will only make the problem worse. Depending on the depth of the stain, you might try sandpapering the area. If this does not work, you will need to seal the stains prior to decorating, (perhaps with an oil based primer) or they will stop the paint from adhering to the surface and/or come through the finished coat.