I did not know that oils could be used for weathering! Wouldn't acrylics be just as useful for this type of work? Is there something special about oils that we'd want to use them here instead of acrylics? You are a true artist, Martin...that loader looks so real!!!
1) The thinner for acryllics is partly water and then alcohol (often isopropanol). If you put acryllic wash on a flat surface and let it dry, you will notice that the paint solved in the thinner will have a tendency to move to the perimeter of the wetted surface when drying. This will in practice mean that a portion of the paint will form a ring outside the actual area where you intended to have your effect. This is not a big issue on a dark surface, like a steamer, or old brown goods hoppers, but a real killer for the effect on yellow, orange, red or white surfaces.
2) The acryllic will harden fast and thereafter not be possible to fully remove, move or change, unless your working on a clean metal surface. This means your bright colored rolling stock will always have the imperfections in the weathering. This problem is most covered by wiping the surfaces with a cotton bud before it fully cures. This however has the drawback that the surfaces most often will be too dark compared to the prototype.