Oil companies are slugging it out with big claims about gasolines that deliver cleaner air, cleaner engines and higher octane - but the octane part of those claims may be meaningless for your car.

Buying a higher octane fuel than your car can use without knocking is simply wasting money, according to an article in the current issue of Popular Mechanics, and buying premium gas will only do your new car good if the blend has some other benefit not available in less expensive grades.Right now there are at least nine oil companies offering reformulated gasolines that promise not only higher octane but cleaner engines and lower emissions.

One reason is that a House-Senate conference committee is looking at a broad revision of the Clean Air Act that could result in restrictions on the sale of gasoline in areas with the poorest air quality and require carmakers to come up with alternative fuel vehicles. Oil and auto interests have sizable investments supporting traditional automobiles.

So 14 oil companies and the Big Three domestic carmakers have joined forces in a multimillion-dollar oilauto conference to evaluate a variety of gasoline blends for current and future cars and trucks, with the target of reducing emissions that are harmful in themselves or those that may contribute to smog.

One of the problems is that there are some 130 hydrocarbons produced when gasoline is burned. The carmakers want to know which have a direct impact on smog formation and that the new gasolines do not compromise vehicle performance.

The reformulated gasolines are just that - reformulations. There are no wonder ingredients or technological breakthroughs. The refiners are reshuffling the mix of hydrocarbon components processed from crude oil that make up their gasolines.

Then they add detergent packages and other ingredients to give desirable properties such as retarding gum formulation and a reduced tendency to absorb water.

Gasoline is a complex product, however, so refiners must be careful. Tinkering with the blend for lower emissions, for instance, might adversely affect octane.

Tailpipe villains on the emissions "hit list" include carbon monoxide, benzene, oxides of nitrogen and hydrocarbons. Hydrocarbons also evaporate into the air when gasoline is stored, pumped into cars and passed through the fuel system. Most cars have fuel evaporative systems to control this, but the volatility of some fuels in hot weather is enough to overpower them.

Reduced vapor pressure gasoline is one solution in summer when high temperatures and the sun promote photochemical smog formation. Some oil company spokesmen admit the product may cause hard starting problems on cold mornings. Benzene, a carcinogen, is another pollutant oil companies are trying to control.

Before the new gasolines were cleaning the air, the old ones were cleaning up your carburetor. Some unstable components in gasoline tend to form gum when exposed to a hot surface. Early detergents added to the fuel attacked dirt and gum in the carburetor with a dispersing action similar to laundry detergent.

A new problem - injector fouling - cropped up in 1985 with the introduction of port fuel injection. Since fuel injectors are closer to the hot combustion chamber than a carburetor, the detergents in use were essentially vaporized before they could attack surface coatings.

New detergents were developed to clean fouled injectors, but they caused problems by carrying dirt down to intake valves. The new detergent additives are there to solve these problems.

Another benefit touted by the new gasolines is higher octane ratings. Octane is simply the ability of gasoline to resist knocking in an engine. There is no reason to buy a higher octane fuel than your car can use without knocking.

The other benefits of the new gasolines, Cliff Gromer wrote in Popular Mechanics, are being made available in the lower octane grades.