Blacker than black oil

[cwshumsky]

I have this little 2005 Isuzu COA city truck 5.2 TD, always wanted to build a mini ramp service truck.. some like souping cars, some like fabricating work trucks:)
This had 100k on the clock when I purchased, every 5k for 2 years I use nothing but Mobil Delvac 15/40 at 150k the injectors were done, my mechanic hands were stained right through the gloves! Never seen oil so black and everything is thickly coated. Since the purchase, the oil was always a little black (It does have an EGR). Maybe the injectors were loading at idle and fouled the oil?
Now, after the injectors were done, the oil after 1 day changed is blacker than black. I want to clean the crankcase and hear filling with diesel is the trick... but I don't want to wash the bearings though.
What is the safest way to do this? Getting back to brown oil is an improvement!