Image: Man changing oil © Ron Chapple, Getty Images

Here's a secret that mechanics don't want you to know: You really don't need to have your oil changed every 3,000 miles.

It's a waste of a precious resource -- not to mention money -- to take your car in every 3,000 miles or three months, experts say. Most cars don't need an oil change for 7,500 miles.

"The oil change itself is a loss leader," said Austin Davis, whose family has been in the car-maintenance business in Houston since 1937.

He's the author of "What Your Car Mechanic Doesn't Want You to Know" and runs the website MyHonestMechanic.com.

"Most repair shops will lose money or at best break even on a $25 to $28 oil change," he said. "The whole idea is to get you to also buy an air filter, rotate your tires or buy something else while you're there."

Complaints about auto repairs consistently rank among the top grievances filed to state attorneys general, according to the National Association of Attorneys General.

Because car manufacturing has become so sophisticated and less reliant on human intervention -- more computers and technology are producing and installing parts, for example -- the car-repair business isn't as robust as it was 10 or even five years ago.

"The easiest way to make up for money that you're losing or to increase profits is to turn up the upsell button on all your services," said Philip Reed, the senior consumer-advice editor for Edmunds.com. "Mechanics want you to get brake jobs earlier than you need them or change oil filters more frequently."

Sometimes, however, we are our worst enemies when it comes to explaining what is wrong with the car and giving away too much information. "Never reveal your budget," said Davis. "If there's steam pouring out of the hood of your Mercedes, don't tell the guy 'I hope this isn't going to cost me $2,000.'

"He'll be thinking, 'How about $1,995,'" he said.

There are no hard-and-fast rules about maintaining cars because there are so many differences in make, model, use and other factors. But experts do agree on this: You should use your car manual as your guide. It will tell you at what mileage mark the oil should be changed or the transmission fluid flushed, what intervals maintenance should follow and a host of other upkeep tips.

"If there's a conflict between what the owner's manual recommends and what the dealer recommends, follow the owner's manual," said Reed. "The manufacturer made the car; they should know what it takes to maintain it and keep it running."

Pay attention to the warranty packages. Cars known for dependability will guarantee parts for as many as 70,000 miles. That's almost the equivalent of driving around the earth three times.

"Cars today are just so well made that the failure rates of parts is close to nil," Davis said.

But long before you need to turn the keys over to a mechanic, find one who is trustworthy and with whom you can build a long-lasting relationship.

"If you develop a relationship with your mechanic, you're much less likely to be ripped off," said Brandy Schaffels, the content manager for the TrueCar website. "They'll go out of their way to help you." She had a mechanic who built an air-conditioner compressor by hand at a substantial savings over buying a new one.

"If your instinct tells you that what they're telling you doesn't sound right, double-check it with another mechanic," she said.