Many automobile owners are spending more than they need on motor oil, believing that it should be changed every 3,000 miles even though almost no manufacturer requires such an aggressive oil-change schedule.

The long-held notion that the oil should be changed every 3,000 miles is so prevalent that California officials have launched a campaign to stop drivers from wasting millions of gallons of oil annually because they have their vehicles serviced too often.

"Our survey data found that nearly half of California drivers are still changing their oil at 3,000 miles or even sooner," said Mark Oldfield, a spokesman for the California Department of Resources, Recycling and Recovery, which has launched the Check Your Number campaign to encourage drivers to go with the manufacturer's recommendations.

Improvement in oils, friction proofing and car engines have lengthened the oil-change interval, typically 7,500 miles to 10,000 miles for most vehicles.

Changing motor oil according to manufacturer specifications would reduce motor-oil demand in California by about 10 million gallons a year, the agency said. The state has created a website, checkyournumber.org, where drivers can look up the suggested motor-oil change interval number for their vehicles.

The agency and other groups said slashing motor-oil consumption would be good for the environment and won't hurt the longevity or reliability of autos.

"Drivers have a number of ways to reduce the environmental impact of their vehicles, which can also save them money," said Don Anair, senior engineer at the Union of Concerned Scientists.

Besides following the manufacturers recommendations for oil changes, drivers can reduce oil consumption by keeping their tires properly inflated and by avoiding idling their engines, Anair said.

"The 3,000-mile oil change just says that the marketing campaign by quick-lube companies has been effective," said Steve Mazor, manager of the Auto Club of Southern California's Automotive Research Center. It made sense years ago, when "we had cast-iron block engines with cast-iron pistons that would expand when they got hot and older lubricants," Mazor said.

Nationally 51% of vehicle owners said they believe oil should be changed every 3,000 miles or three months, according to an August survey by market research firm NPD Group. And just 33% wait more than 4,000 miles between oil changes, NPD said.

Drivers should be confident in the oil service advice offered by automakers, Mazor said.

Vehicle warranties, especially for power trains, have grown longer in recent years, he noted, and automakers would not give advice that could hurt engines and increase their warranty expenses.

Ford Motor Co. recommends oil changes for most of its new vehicles at 10,000 miles, although some still require the service at 7,500 miles.

"Our new generation of engines have tighter internal tolerances, which reduces the amount of carbon and other products from combustion that gets into the oil," said Richard Truett, a Ford spokesman.

The latest engines also run at more optimum temperatures, which diminishes the degradation of oil.

Honda Motor Co. and its luxury marque, Acura, no longer have a set interval for motor-oil changes.

Both Honda and Acura vehicles are equipped with a maintenance minder system that recommends oil changes and other services based on a number of vehicle-usage factors, including mileage and climate. Other manufacturers have similar systems that alert drivers to the need for an oil change.

"The idea is to prevent either over- or under-maintaining a car by following a set schedule," said Chris Martin, a Honda spokesman. "Now, there is no guesswork."

Typically, the indicator for an oil change lights up every 5,000 to 10,000 miles in Honda and Acura vehicles.

Owners of late-model BMWs can go as many as 15,000 miles between oil changes, depending on driving conditions. BMWs also have sensors alerting drivers to the need to change motor oil based on conditions like driving in stop-and-go traffic, making short trips and prolonged idling.

Despite these improvements, many drivers insist on changing the oil often even if it isn't recommended.

This has prompted some new car dealers to ask customers who come in for an oil change when it's not recommended to sign a document stating that they understand that the maintenance guidelines for their auto don't require a change at that time. The dealers are worried that someone might later accuse them of selling unnecessary maintenance services.