LOS ANGELES, California - Many vehicle owners are spending more than they need motor oil, believing that it should be changed every 5000 even though almost no manufacturer requires such an aggressive schedule.The long-held notion that the oil should be changed every 5000km is so prevalent that California officials have launched a campaign to stop the waste of millions of litres of oil every year.TOO MUCH, TOO SOONMark Oldfield, a spokesman for the California Department of Resources, Recycling and Recovery, has launched a 'Check Your Number' campaign to encourage drivers to follow manufacturers' recommendations.He said: "Our survey data found that nearly half of all California drivers are still changing their oil at 5000km or even sooner." Improvements in oils, friction-proofing and car engines have extended the oil-change interval, typically from 12 000km to 16 000km for most vehicles. So some folk are changing there oil three times too often!Changing motor oil according to manufacturers' specifications would reduce motor-oil demand in California by about 37.8-million litres a year, the agency said. The state has created a website, CheckYourNumber.org, on which US vehicle owners can look up the suggested oil change interval for their vehicle.The agency and other groups said slashing oil consumption would be good for the environment but not affect the longevity or reliability of autos.Don Anair, senior engineer at the Union of Concerned Scientists, said in a report carried in the Detroit News: "Drivers have a number of ways to reduce the environmental effect of their vehicles, which can also save them money."Besides following the manufacturers' recommendations for oil changes, drivers can reduce oil consumption by keeping their tyres properly inflated and by avoiding idling their engines, Anair said.IMPROVEMENTS IN TECHNOLOGYSteve Mazor, manager of the Auto Club of Southern California's Automotive Research Centre, said in the Detroit News feature: "The 5000km oil-change just says that marketing campaigns by quick-lube companies have worked."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.In the US, 51% of vehicle owners believed oil should be changed every 5000km or three months, according to a survey by market research firm NPD. Another 33% stretched that to 6500km.Vehicle owners should be confident in the oil service advice offered by automakers, Mazor said. Vehicle warranties, especially for powertrains, have been extended and automakers would not give advice that could hurt engines and increase their warranty expenses.Despite improvements, many drivers insisted on changing their vehicle's oil often even if it isn't recommended, the DetNews said. This has prompted some new car dealers to ask customers to sign a document stating that they understand that the maintenance guidelines for their vehicle don't require an oil change at that time. Dealers were worried that they might be accused of selling unnecessary maintenance services.