It is a common misconception that an oil’s color is an indication of how “dirty” it is. This is not true. It is often a common tactic used at quick lubes and service centers; the technician pulls the dipstick and wipes it on a white shop cloth and shows the customer how “black and dirty” it is. Any oil will turn black after a short period of use. Some oils may stay “clean” looking longer than others, but eventually they all will turn black. This is perfectly normal.
When someone tells me how “clean” their oil is because they have pulled the dipstick and it looks clean I always tell them that it will eventually turn black. They also tell me when they pull the dipstick and it has becomes black and “dirty” it will require changing. That’s about the time I will pull my dipstick in one of my trucks and show them how black and “dirty” the oil is. I will then produce my latest oil analysis test report that provides laboratory chemical and spectrographic test data confirming that the oil perfectly suitable for continued service.
In general, the color of an oil does not have any bearing on its lubrication ability or whether or not the oil is suitable for continued use*. Most oil and especially diesel engine oil will turn black in the first few hours of operation due to contaminates generated by the combustion process and soot particles. It is the job of the filtration system to filter out the larger sized soot particles that can cause engine wear and the additive package of the oil to neutralize and hold in suspension the soot particles that are too small for the filter to trap and hold.
* Under certain conditions such fuel dilution, water contamination or glycol contamination, for example, the color can provide insight that something is mechanically wrong and in need of repair and/or additional analysis, however under normal operating conditions without mechanical problems present the black color which is commonly referred to as “dirty oil” in the vehicle servicing industry does not have any bearing on its lubrication ability.
The only way to accurately determine an oils lubricating value or contamination level is through (spectrographic) oil analysis. Oil analysis is common practice used regularly in commercial, industrial and fleet operations and can also be used for passenger cars, light trucks or any other application.

The Dark Oil Myth
Dark oil does not indicate the need for an oil change. The way modern detergent motor oil works is that minute particles of soot are suspended in the oil. These minute particles pose no danger to your engine, but they cause the oil to darken. A non-detergent oil would stay clearer than a detergent oil because all the soot would be left on the internal engine parts and would create sludge. If you never changed your oil, eventually the oil would no longer be able to suspend any more particles in the oil and sludge would form. Fortunately, by following the manufacturer's recommended oil change interval, you are changing your oil long before the oil has become saturated. Remember, a good oil should get dirty as it does it's work cleaning out the engine. The dispersant should stop all the gunk from depositing in the oil pan.
The only real way to determine whether oil is truly in need of changing is to have an oil analysis performed. Since most people don't want to bother with this, it's acceptable to err heavily on the safe side and simply follow the manufacturer's recommended change interval for severe service. There are still a few cars that specify 3K intervals for severe service, but not many. If you look at countries other than the U.S., the oil recommended change interval is much higher than even the normal interval specified by vehicle manufacturers in the U.S.