I am really new to Diesel engines.. I did the oil change at the local dealership on Monday. Right after I got the truck back, I checked the oil and the oil had already changed to black color. It was not as black as when I gave it to them. Besides, the overhead still says "oil change required". Why did the color change black so soon...?? Is this bad? Does this mean that the engine may have been abused by the previous owner (I am the second owner and I just bought this truck)? Thanks for the answers.