When engine oil turns dark, it's dirty and should be changed.
If you're conscientious about keeping your car in good running order, you probably worry from time to time that your oil has gotten dirty and is causing sludge to build up in your engine. So you pull the dipstick out and check the color of the oil at the tip. Chances are, it's starting to turn dark, no longer the light amber color that you saw on the stick when your oil was fresh. So now it's too dirty to use, right? It's depositing sludge in your engine and needs to be changed.
Wrong. In fact, just the opposite is true. If you're using a detergent engine oil (and most modern engine oils have detergent additives), the oil is working just the way it's supposed to, dispersing the tiny particles that can result in engine sludge and holding them in suspension in the oil itself so that they can't build up. That's why the oil appears darker, but this in no way impedes the oil from performing its normal functions of lubricating and protecting the metal surfaces inside the engine. Of course, the oil is limited in how many of these suspended particles it can contain and will eventually need to be changed when it becomes saturated, but use the oil change interval recommended by your car's manufacturer to decide when to change the oil, not the color of the oil on the stick.