There's nothing wrong with diesel oil per se but it's formulated with additives that keep the soot that diesels are prone to, in suspension. It will do the same on your engine but most of the black stuff is probably being washed off the insides of the block etc as its also a good cleaner. No bad thing, but it'll clog up your oil filter quicker, so extra changes a good idea. If the oil starts to look a bit cleaner I'd swap back to a regular, but high zinc oil.