Oil Change Intervals
Many people feel that 3000 mile (5000 km) OCIs are necessary. This notion may be a hold-over from early maintenance practices and is definitely reinforced by modern marketing. Obviously, an oil change shop will make more money if you bring your car back to them more often. Even in 1965, the OCI recommended by the factory for Plymouth Valiant normal service was 4000 miles (6400 km) and this was with a API MS/SC engine oil. Modern oils, especially synthetics, can easily go much more than 5000 miles (8000 km) between changes with non-severe driving. To save money on unnecessary oil changes and wasted time, use a synthetic oil and time your oil changes to coincide with tire rotations. Tire rotations are commonly done every 10,000 km or every 5000-6000 miles. Newer cars equipped with oil life monitoring features such as the GM Oil Life Monitor (OLM) can safely drive their vehicles until the OLM indicates an oil change is due.
For those people using conventional mineral engine oil, the recommendations found in your owner's manual should be followed. Although many oil manufacturers tout their products' ability to have extended OCIs, it probably makes the most sense to time your oil changes to coincide with other scheduled maintenance so as to minimize the vehicle's downtime as well as your time.
Used Oil Analysis
The only way to truly know whether an engine oil has reached the end of its lifespan is to have a sample of it analysed by an oil testing lab. It is normal for an oil to become dark because the detergent/dispersants are keeping contaminants in suspension. Oil analysis will indicate whether the additive package has been depleted and measure the contaminant load in the oil. If you want to significantly extend your OCIs, please consider using this service. See What is Oil Analysis? for more information.