Heat cycles from the engine will cause your Car's Engine Oil to Darken. The more Extreme Heat you drive in and the more frequent you have Heat Cycles, the quicker your Oil will Darken. A Heat Cycle is when the Engine starts cold, hits operating temperature and then cools back down. Oil will turn darker over time due to Oxidation. Oxidation is when the Oxygen in the air interacts with the oil and causes it to chemically breakdown. Heat will speed up the process of Oxidation. Soot however will cause the oil to turn Black. Every modern Engine Diesel or Gas will produce Soot. Soot is a byproduct of incomplete Combustion. The longer you go between Oil Changes the more Soot you Oil has suspended in it. Soot is generally too small for your oil filter to filter out. If you accumulate too much Soot, the Soot will lump into larger groups and will cause Engine Wear. Some Oils Darken quicker than others due to the different Additives they have in them. So Color can be misleading. The only way to be 100 percent sure. Have your Engine Oil Chemically analyzed over different intervals. Or to be on the safe side have your oil changed on regular intervals based on Manufacturer Specification. Have one of the Trained Service Advisers at Stamps Auto Tire Pros help you with this.