The International Energy Agency, which represents the world's principal oil-consuming countries, today revised upward its projections for oil demand during the first quarter of 1988.

The IEA Monthly Oil Market Report is now predicting that 50.2 million barrels a day will be consumed during the first quarter in the countries it tracks, which includes all but those with centrally planned economies.

Previously the agency had forecast consumption of 48.9 million barrels a day.

Although the revision is substantial, it is not out of line with other available forecasts, said the IEA's oil division chief, Marcel Kramer.

The report also increased the agency's projections of consumption in 1987, which suggests that oil inventories may be somewhat smaller than previously assumed.

The revised data, combined with the blast of Arctic weather that blew across the United States last week, may provide some support for oil prices, which have been depressed since the Organization of Petroleum Exporting Countries met in December and failed to reach an accord that would result in reduced production.

"The temperature is definitely a positive factor," said John Lichtblau, president of the Petroleum Industry Research Foundation.

The colder temperatures during the first days of the year may help offset warmer-than-normal weather in December and result in a reduction in heating oil stocks.

But economists and oil industry analysts said Friday that the key to improving oil prices is production -- "The key to what happens to prices is restraint on supplies." Sanford L. Margoshes not consumption. Lichtblau noted that OPEC production continues at relatively high levels, despite reductions in December and in the first eight days of January. "If they continue to produce at 18.3 million barrels a day or anything like that in January or February, they are overproducing," he said. "You can't have 18 million barrels a day and {oil at} $18 dollars a barrel. The two don't go together."

OPEC stuck to its $18 a barrel price at its meeting in December, despite entreaties from Iran and a handful of other members to raise the price. Since that time, however, oil has been trading at prices well below that level, reaching as low as $14.50 in recent weeks. The price of OPEC crude fell 25 cents to $15.25 a barrel on the European spot market Friday.

"The key to what happens to prices is restraint on supplies," said Sanford L. Margoshes of Shearson Lehman Brothers. Margoshes said, however, that he anticipates some short-term strengthening in oil prices.

Saudi Arabia, having rejected Iran's demands for a higher price, will seek to support the $18-a-barrel price, he said. In addition, Margoshes said he anticipates more attempts to cut back production, some reduction in heating oil stocks because of cold weather and possibly renewed hostility in the Persian Guild to strengthen prices.

He also noted that "when the pessimism about crude oil prices is rife, it's usually the time to plant the seeds of recovery."

The IEA's first-quarter revision represents a better analysis of oil consumption in developing countries, according to the Paris-based agency. The report noted that developing countries account for a rising share of world oil consumption -- more than 20 percent in 1986, compared with 13 percent in 1973.

Another report released today indicates a major shift in the structure of international oil trade, with the spot market accounting for a growing share of imported U.S. crude supplies. Single-cargo spot transactions now account for nearly 40 percent of U.S. crude imports, according to the Petroleum Intelligence Weekly.

The volume of those transactions has more than doubled since late 1984, according to the publication, which based its conclusions on U.S Department of Energy data.