For some people, it's hitting the big five-oh that really hurts -- that is, dropping $50 on a tank of gas. For others, it's just that relentless upward creep in prices that gets their attention.

Whatever the trigger, drivers pulling up to the pump in vehicles that ostensibly require high-grade gas are wondering if they really need the more expensive fuel or whether it's okay just to fill it up with regular. As gas prices soar, car owners increasingly are going for the cheaper stuff -- no matter how fancy their wheels. And station owners and oil companies are seeing the impact: Sales of premium and mid-grade gasoline are tumbling.

It's an age-old response, industry experts say, for drivers to switch from pricey, higher-octane formulations of gas to cheaper alternatives whenever gasoline prices rise substantially. Now, with prices stuck stubbornly high, oil experts wonder whether high-grade gas will go the way of the Studebaker.

"I foresee no serious decline in prices anytime soon, so the question is, will consumers' buying habits change permanently if the higher prices stay as they are," said Daniel F. Gilligan, president of the Petroleum Marketers Association, which represents independent filling stations. "Will it be more difficult to attract consumers back to the higher-octane fuels? I don't know."

Automotive experts say using regular gas in most vehicles does no damage and makes no discernible difference in performance. Cars made in the past 15 years have such highly refined computer controls that the engine will adjust to the grade of octane in the gasoline, even in cars sold as requiring premium gasoline. Some drivers -- in some cars under some driving conditions -- may notice a drop in horsepower, but for most people behind the wheel, it wouldn't be enough to notice, the experts say.

"It's not going to hurt anything," said Peter Gregori, service manager for EuroMotorcars, a Mercedes-Benz dealer in Bethesda. In fact, Gregori has been using regular gas in one of his own Mercedes cars for two years, and "it's perfect," he said -- even though Mercedes-Benz says owners should use only premium.

"I get better mileage with the regular than I do with the high-test, in this particular model that I have," Gregori said. Among cars that come in for service, Gregori said, he can't tell which have been sipping premium.

Apparently, drivers are figuring this out.

Nationally, sales of premium gasoline fell 5.6 percent this year through May, the latest data available, according to the Energy Information Administration, while overall regular gas sales were up 4.7 percent.

Premium sales were down almost 9 percent in Virginia and 5.5 percent in Maryland.

In the District, premium sales were down 28 percent, an especially steep decline that may be overstated because of a fluctuation in suppliers reporting for such a small market, EIA economists said. But the relationship between regular gasoline, which fell 22.5 percent, and premium sales is accurate, said economist Michael Burdette.

"Volume-wise, we're definitely selling less," Chris Wangkang, manager of a Shell station in Rockville, said of the station's premium V-Power gasoline. As he spoke, a young woman in a black BMW -- another carmaker that recommends premium fuel -- pulled away from the pump outside.

"What did she get?" Wangkang asked his cashier, who checked her receipt.

"Regular," he replied.

The first drivers to defect from premium, station managers say, are those whose cars don't specify a need for premium gasoline but who were using it anyway, thinking it might help -- even though most experts say it won't. But even owners of cars that purport to require high-test -- complete with a warning on the gas cap, "Premium Fuel Only" -- also are trading down.

David Shapiro said he and his wife switched to regular independently of each other about two weeks ago. It was a spur-of-the-moment decision for the tax lawyer with the D.C. office of PricewaterhouseCoopers.

"I never really knew if it made a difference, and I figured it was time to try and see if my car really works fine," he said of his Volvo, which recommends premium fuel. "It's like two bucks a tank difference, so it's like a free gallon. And it hasn't made a bit of difference in how the car drives."

Although the numbers are slightly different in some states, premium fuel typically has an octane rating of 91 while regular fuel's rating is 87. Octane controls the fuel-air mixture in an engine to keep it from igniting before it's supposed to, which can cause reduced power and harmful knocking. But in all modern cars, computers adjust the timing of the engine's compression so that "pre-ignition" never happens, said Cole Quinnell, a spokesman for Chrysler Group Engineering.

"It does not hurt the engine -- sensors automatically readjust the engine basically for that reason, to save itself," he said.

Chrysler recommends 91-octane fuel for its high-performance and turbocharged automobiles, such as the SRT models, "but all our products will run on 87 or 89," Quinnell said. Any loss in performance would come when an engine is pushing to its maximum power and speed, he said, so "if an average driver got into one of our SRT vehicles, they wouldn't notice any change."

Karl Brauer, editor in chief of Edmunds.com, an automotive information and research firm, said carmakers perpetuate the premium fuel requirement because engines designed for that gas can achieve greater performance when powered by the high-grade fuel.

"Manufacturers want to be able to quote high horsepower numbers," Brauer said. "If it's not run on premium, it probably won't ever hurt the car. It probably won't ever be noticed by most drivers. But they can still have their claimed horsepower for their brochures."

Oil refiners make the most money from selling higher-grade gasoline because the higher cost to produce it is more than made up by the added charge to customers, said Fadel Gheit, an oil analyst at Oppenheimer & Co. in New York. Gas stations also benefit from higher margins on premium fuel.

"It's to everybody's advantage to push the high-grade," Gheit said.

For their part, the oil companies say they're just providing the fuel that carmakers demand. "We're guided by what the car manufacturers say and what the octane requirement would be, so our responsibility is to fill the need and provide what the consumer would require for their cars to work most efficiently," said Gerald T. Davis, a spokesman for Sunoco. Asked if premium gasoline is more profitable for the oil company, Davis would say only, "Our gasoline is priced competitively and fairly."

Staff writer Justin Blum contributed to this report.