I don't recommend engine flushes. I prefer to pull the oil pan and clean it up myself so I can inspect the possible damage. The main parts of the oiling system are the oil pump, filter, passageways and bearings. At this point I would pull the pan, and inspect the bearings on the crankshaft and connecting rods. Maybe the bearings are worn causing lower pressure.
Does the light go off when the engine is above idle? If so does it stay off always when raised above idle? If so, I would think it is the bearings being worn. Although when engine problems like this, I would recommend that you do not drive the van. You risk siezing the engine if the oil pressure is low.