dirt in your oil ? are you not using a filter LOL sorry had to .. yes oil get contaminated but its the additive that break down is the real reason for changing your oil .. i wont argue with people that change there oils more frequently but if your car is in tip top running condition your oil should be good for 5-6000 miles depending on driving conditions as your service manual states . oil and cars have come a long way . i run synthetic and let it go about 8000 miles before my change