I would experiment with the oils as you suggested, and possibly some pigments near the bottom after the oils are dry. Since the overall weathering in the photo you've linked to seems to be in a vertical direction as opposed to horizontal in the direction of airflow, as well as being dirtier on the bottom, this suggests that it has been left outside in the damp elements. Dirt and mud has accumulated and either light rains or dew has slightly washed the dust and dirt in off in the direction of gravity, sort of like with armor. There also seems to be a subtle mottling and light splattering which is also more apparent towards the bottom.
Just my observations, I could be completely wrong...
George