If You Don't Change the Oil
Most likely: You know you're supposed to change your car's oil every 3,000 miles, but sometimes you just can't get to Jiffy Lube right away. Luckily, even the lube dudes who have a vested interest in this ritual admit that you can get away with changing the oil at 7,000 or 8,000 miles. "This rule is something formulated by the oil-change industry," says Automobile Club of New York manager of public information Robert Sinclair Jr. "It's wasteful."
Worst case: If you wait until 10,000 miles or so, your oil may turn to sludge and your engine might seize. If you willfully ignore the knocking under your hood, you probably deserve to pay the $2,000 (or more) to replace your engine.