Blacker than black oil

[Rawze]

The least expensive thing to do is a lab analysis and find out why it is so black. I would not use diesel in it unless you are getting ready to tear it down to overhaul it. Instead, I would change the oil a few times in a row, like every month for about 3 or 4 oil changes and get all the old soot and carbon buildup out of the engine slowly and safely. You also should consider an oil that was designed to handle a lot of soot, like Rotella-T instead of the mobil, and if your going to keep the EGR, a bypass filter is your first defense towards keeping the engine alive.

Should investigate how much egr gas its using and how much soot its making. If it is that bad, your muffler and exhaust is likely suffering flow restrictions too. Check that the EGR valve is not opening too far and replace sensors that may be reading wrong.

There are reasons why engine run poorly and make too much soot. Most people ignore it until it has already taken its tool on everything but it is bad practice.