Demand for unleaded gasoline in the United States shot up by 15.3% in September from year-earlier levels, the American Petroleum Institute reported. The nation's total oil demand climbed 3.7% last month as imports dropped 1.6% and domestic crude oil production fell 2.9% from the September, 1986 pace, the oil industry trade group said. Overall gasoline demand climbed by 4.3% to 7.15 million barrels a day in September from 6.86 million barrels a year earlier. The API attributed robust demand for gasoline to "the continued impact of prices still roughly one fourth lower than two years ago, before last year's major price decline." For the first nine months of 1987, gasoline demand rose 2.5% from the year-earlier period. U.S. demand for all petroleum products increased to 16.49 million barrels a day last month from 15.90 million barrels a day in September, 1986.