Dark Oil. Dark oil can mean multiple things as well. It could simply just be darkened because of the additives. Or, the darker shade could result if the oil has also been cycled through too many times, and it’s time for a change. If the oil is thick and dark, it could indicate dirt or contaminants, especially if it’s an off-road rig. Within the ethylene industry, "Red oil" is an organic contaminant frequently encountered in caustic towers. The "red oil" is an organic polymer that forms from the aldol condensation of acetaldehyde in sodium hydroxide solution. Initially, the acetaldehyde forms a light floating yellow oil. The engine also breathes air and air is dirty, the filter catches most of it however some makes it into the combustion chamber and also to the oil. Dirty oil is not bad though. that means the detergents in the motor oil is doing what it was designed to do. on an engine with over 15,000 miles I would get worried if my oil came out clean after being run in the engine.