American motorists are turning increasingly to super unleaded gasolines, many of them unnecessarily. This year they will spend nearly $700 million more than they would if they used only the standard unleaded, and, according to one major oil company's estimate, about 20 percent of that may be wasted.

About 10 percent of all gasoline sold this year will be super unleaded, up from 6 percent in 1980 and 21/2 percent in 1978, according to Dan Lundberg, editor of the Lundberg Letter, which follows petroleum marketing. Lundberg estimates that super unleaded, normally 91-93 octane, will be 30 percent of all sales by 1990.

Automotive and oil industry experts offer a variety of complex explanations for the surge in the sale of super unleaded gasoline, which generally costs 3 to 10 cents more per gallon than regular 87 octane unleaded:

Noise intolerance may be the biggest reason consumers have dug into their wallets to pay for super unleaded gas. Experts insist that occasional knocking and pinging, which occur if fuel ignites too soon in an engine's cylinders, are not harmful to the engine. But when many car owners hear a knock going up a hill, they pull up to the super pump where higher octane silences the noise.

Oil and auto industry experts agree that a large number of those customers -- Shell Oil Co. puts the figure at 20 percent -- are wasting their money, and wasting oil supplies, since refiners get less super unleaded gasoline from a barrel of oil than regular unleaded.

The recent tendency to postpone new car purchases has kept a lot of cars that use unleaded gas on the road longer than expected. These cars are subject to octane creep--as they get older, they need a higher octane unleaded or they tend to knock. (Octane is defined as a measure of gasoline's ability to resist knocking.)

A certain percentage of new cars, virtually all of which manufacturers say are designed to run on 87 octane, actually need a higher one because they don't come close enough to design requirements. Ford, for example, estimates that is the case with something under 5 percent of its cars.

A few oil companies lowered the octane in their regular unleaded when they introduced a super product. Some consumers whose cars used to run fine on the cheaper unleaded, lost performance without those few extra octane points, and switched to the more costly super product.

Since premium leaded gasoline is virtually extinct, owners of big, old high-compression-engine cars have been mixing their tanks with super unleaded and regular leaded to get a higher octane average.

All types of unleaded gasoline now represent more than half of total American gasoline sales, a result of a requirement that all cars made in the United States since 1975 have catalytic converters. These emission control devices require unleaded fuel because they can be rendered useless by leaded gasoline.

Clarence Ditlow of the Center for Auto Safety, an independent consumer group, argues that by promoting higher octane unleadeds, auto companies are "trying to replace their old profit leader, premium leaded, with a new one, a premium unleaded."

Ditlow also faults the oil companies for eliminating what he calls midgrade gasolines. "In the '70s," he said, "there were three levels of gasoline--the 89 regular, 93 midgrade and 97 premium. Now they have gone to a two-grade system for unleadeds for the most part." The oil companies' response to this is that most station islands only have three pumps, and one must still have leaded gasoline. Sunoco is one of the few major companies offering a midgrade (89 octane) unleaded locally.

Ditlow, pointing out that the price differential between leaded regular and premium unleaded "can be 10 cents when the octane is only two points higher on the unleaded," says that "is outrageous. Most people don't know too much about the octane system so when they knock at 87, they go to a 91-93 but they may only need 89 to operate well."

"A certain amount of people are overbuying and the net result is a waste of fuel," said Richard Kozlowski, director of the field operations division, office of air programs, at the Environmental Protection Agency. "We don't see a major problem because there is plenty of fuel around, but some people might be wasting it....The people concerned with economy are willing to take an occasional ping."

American automobile manufacturers don't knock pinging.

"If your car pings, we call that the sound of fuel economy," said Ford's Robert H. Harner.

The folks at General Motors agree. "A certain amount of knock is pretty good. It means you are coming as close to a full fledged explosion as you can," said R.T. Kingman, GM public relations director, who uses only regular unleaded gas in his Monte Carlo and Skylark. He said the occasional pings don't bother him.

"If your engine is tuned for optimum fuel economy, you're on the borderline of knock," said Chrysler engineer Tony Bierylo. "You'd have to have a constant, very audible knocking noise for about 1,000 miles of driving before you could damage your car. Then it could possibly knock a hole in your pistons. But it would have to be very severe."

"There would be money wasted by a customer who would be buying super premium if they were just intolerant of a little knock," added Bierylo. "Just don't get it to satisfy your hearing. But, it all depends on what you want out of life. If it's worth this money to you, it's worth it."

Some car owners buy costly high octane gasoline because they feel it is beneficial for their car or they think it can tide them over until their next tune-up. "Some people would rather pay for premium gas than pay for a tune-up," said GM's Kingman. But "if you need a tune-up, even 100 octane gas won't do your car any good," said Bob Shelton, who owns a Georgetown Gulf Station.

"Consumers ought to understand that it is worth nothing to buy high octane when the car runs without knocking," said Jack Blum, general counsel for the Independent Gasoline Marketers Council. "The only thing to look for in gas is the price and whether or not the car knocks."

Robert Jewett of Shell Oil Co. says, "Our advice to any customer is to fill up with the 87 octane. If you car runs fine, stick with it. If you get engine run-on and knocking, then you do have to upgrade. If you really don't need the higher octane, it's wasteful from an energy standpoint."

With that in mind, Shell's current TV campaign is urging people, "If you don't need the difference, don't pay the difference."