Ah, if the question were only that simple. Yes, there are differences in brands of gasoline, but then again, there aren't. What's safe is a two-part conclusion. All gasoline sold in the United States is basically the same stuff, mingled near the distribution point in the same gigantic tanks, and it is all of good quality. But when the gasoline rivers branch off to be pumped into separate trucks, companies tinker with it, taking that consistently good product (whether regular or premium) and spicing it with their own chemical signatures, the additives that they claim set them apart. The resulting differences are hardly major ones, but if you're an aficionado tuned to your engine's every ping, you may care about them.

Michael Scardino of Brentwood, Tenn., a 52-year-old advertising creative director, said his cars ran better on Chevron and added, ''I'd go out of my way to get it.'' But when the brand is Shell, he said, ''I'm never happy with the performance.'' He also switches periodically between Chevron, Citgo, Exxon and Texaco ''to get the additives from one that may be missing in another brand,'' he said.

Meanwhile, Laura Brengleman, 41, who lives in Greenwich, Conn., and is the editor of the motorcycle magazine RoadBike, puts the cheapest gas she can find in her family car, which has rolled up 150,000 miles so far. ''I have no problem buying from a no-name gas outlet,'' she said, ''as long as I see it's a busy station with turnover.'' (When a station goes through its gasoline supply quickly, there's less potential for water to condense from the air and settle into half-empty tanks.) On the road, Ms. Brengleman said, she might look for Exxon because she counts on its restrooms to be clean.

It sounds as if this brand issue is one for the experts. But they're not much help.

''I can't say one brand is better than another,'' said Dr. Edward Murphy, a spokesman for the American Petroleum Institute, a trade group that represents oil companies in Washington. ''Base gas is a freely traded commodity that must meet certain government specifications. It flows through common pipelines into commingled storage tanks.'' What he could say, he added, is that ''the major brands spend R-and-D money on patented additives, which they add to the basic gas.''

Ron Baker, the director of the Petroleum Extension Service of the University of Texas at Austin, is well informed on these additives, which are the true distinguishing elements of gasoline brands. He described them as ''detergents and other chemicals'' designed to keep gasoline from evaporating, keep engines from corroding, help engines run well in different climates and seasons, and keep engines clean. The goal, he added, is ''forming less deposits when the gasoline burns, for better mileage, performance and exhaust emissions.'' So which brand does Mr. Baker choose? ''I buy cheap gas,'' he said. ''All U.S. gas is of good quality.''