The 3,000 Mile Oil Change Myth

Yahoo Autos

By Bill Siuru, Greencar.com

According to a recent study by the California Integrated Waste Management Board, 73 percent of California drivers change their oil more frequently than required. This same scenario no doubt repeats itself across the country. Besides wasting money, this translates into unnecessary consumption of $100-a-barrel oil, much of it imported.

Using 2005 data, the Board estimates that Californians alone generate about 153.5 million gallons of waste oil annually, of which only about 60 percent is recycled. Used motor oil poses the greatest environmental risk of all automotive fluids because it is insoluble, persistent, and contains heavy metal and toxic chemicals. One gallon of used oil can foul the taste of one million gallons of water.

It’s been a misconception for years that engine oil should be changed every 3000 miles, even though most auto manufacturers now recommend oil changes at 5,000, 7,000, or even 10,000 mile intervals under normal driving conditions.

Greatly improved oils, including synthetic oils, coupled with better engines mean longer spans between oil changes without harming an engine. The 3000 mile interval is a carryover from days when engines used single-grade, non-detergent oils.

For several years, automakers like General Motors, BMW, and Mercedes-Benz have installed computerized systems that alert drivers via an instrument panel light when it’s time to change oil. As an example, the General Motor Oil Life System (GMOLS) analyzes the engine temperature, rpms, vehicle speeds, and other driving conditions to calculate the rate of engine oil degradation. Then, software calculates when the oil needs to be changed. Other systems work similarly.

Because of the many external conditions and parameters that have to be taken into account, calculating the precise maximum service interval using mathematical models alone is difficult. Now, Daimler AG has developed a more direct and precise way to monitor oil quality directly on board a vehicle.

Daimler uses a special sensor integrated into the oil circuit to monitor engine oil directly. Oil doesn’t wear out, but rather dirt and impurities cause oil to lose its ability to lubricate properly, dictating the need for a change. Daimler uses the oil’s “permittivity,” that is, the ability to polarize in response to the electric field. If the engine oil is contaminated by water or soot particles, it polarizes to a greater extent and its permittivity increases.

To evaluate the quality of the oil, permittivity is measured by applying an AC potential between the interior and exterior pipes of an oil-filled sensor to determine how well the oil transmits the applied electric field.

Because not all impurities can be measured with sufficient precision via the electric field method, Daimler also measures the oil’s viscosity to detect any fuel that may have seeped into the oil. Daimler researchers measure viscosity while the vehicle is in motion by observing the oil's side-to-side motion in the oil sump. The slower the oil moves, the higher its viscosity. This movement is registered by a sensor and the viscosity is calculated on this basis.

A single sensor, along with the information already monitored by on-board computers, is sufficient to determine the various parameters of the engine oil. Daimler will likely use the technology first on its commercial vehicles. Here, large oil reservoirs mean larger quantities of oil can be saved. Plus, a predicted 25 percent increase between service intervals and reduced downtime will be of interest to fleets, and thus justify the added cost of installation.