A sharp decline in oil prices would give a strong economic boost to the United States and other oil importing countries, easing inflationary pressures and reducing unemployment, administration officials and private analysts said yesterday.

As signs of an imminent major break in oil prices increased, most analysts stressed the likely benefits. Among them:

* A reduction of $5 a barrel would raise living standards in the United States by almost 1 percent this year, if the full benefits of lower energy costs were passed on to consumers.

* A cut of that size also could cut gasoline prices by 12 1/2 cents a gallon, reduce consumer prices by 1 percent by the end of this year and unemployment by nearly 0.4 percent, according to estimates by some private economists.

But the anticipation of an economic windfall from an oil price war was tempered on two fronts. Some oil analysts said it was unlikely that energy companies would lower fuel prices by as much as the decline in crude oil prices.

And analysts warned of the risk that a steep oil price decline could shock the world financial system, creating problems for debt-laden oil-producing nations and their creditors.

"An oil price decline is not unambiguously good," Secretary of State George P. Shultz told Congress yesterday. But he and other officials said that on balance lower oil prices would strengthen the U.S. and world economies.

The main losers from an oil price decline would be the oil-producing nations, most notably Mexico, and domestic oil producers. Banks that have lent heavily to these could in turn be hurt as the oil price drops, analysts said.

However, one official commented that "even with Mexico" it is "well worth it" to have the benefits of lower oil prices. "The world can come up with more money for Mexico," this source said.

Meanwhile, consumers in the United States and elsewhere would enjoy lower gasoline and other fuel prices, and would have more money left to spend on other things, analysts said yesterday. Companies outside the energy industry would be helped directly by lower energy costs, and indirectly by increased consumer spending.

Auto sales, for example, could be boosted by 300,000 this year if oil prices drop by $5 a barrel, according to a study by Data Resources Inc., a private economic forecasting company.

The United States as a whole, along with other oil-importing nations, would be richer as a result of lower world oil prices, analysts said. Shultz said the 1983 oil import bill for industrial countries would drop by $90 billion if world prices fell to $20 a barrel.

He predicted that such a decline would raise real growth rates in industrial countries by as much as 1 to 1 1/2 percent a year. This would leave the industrialized world growing 1 1/2 times as fast as presently expected, State Department officials said. In turn, "renewed growth in the industrial countries will aid the recovery of developing countries," Shultz said.

For this country, an immediate cut in oil prices of $5 a barrel would raise living standards by almost 1 percent this year and 1.2 percent in 1984 and 1985 if all of the effects are passed on to consumers, DRI estimates.

Consumer prices would be 1 percent lower by the end of 1983 than otherwise, and the nation's total output of goods and services, or Gross National Product, would be 0.8 percent higher, the study said.

Consumers may not receive all of the benefits of lower oil prices, however. Dan Lundberg, publisher of a Los Angeles-based oil industry newsletter, estimates that gasoline prices have dropped by 14 cents a gallon nationwide in the past seven months, eating into profit margins of refiners and marketers. They would likely hold onto some of the initial gains from cheaper crude in order to make up for this, analysts said.

Furthermore, governments, at both the federal and state level, may take some of the benefit of cheaper oil imports away from consumers by increasing energy taxes. A 5-cent-a-gallon increase in gasoline taxes is already scheduled for April, and if the world oil price drops dramatically, further tax increases are likely, analysts say.

Martin Feldstein, President Reagan's chief economic adviser, commented yesterday that if there is a very substantial oil price decline "then I think we have to reexamine more generally our whole energy and energy taxing policy." Energy tax increases would limit the economic impact of any oil price decline, but would help the federal and state governments to reduce their budget deficits.

Industrialized nations such as Japan, West Germany and France would benefit directly by paying less for their oil imports and would be able to grow faster with less inflation, economists say.

The same is true for developing nations such as Brazil, South Korea and Taiwan, which have hefty oil import bills. They would also gain to the extent that lower oil prices spur the world economy, and increase world trade, Shultz pointed out.

Perhaps most important, if a sharp oil price decline triggers lower interest rates, as some analysts hope, debtor nations throughout the Third World would be helped enormously by the cut in the cost of servicing their foreign loans.

There are two caveats to the general welcome that economists give to the prospect of cheaper oil. One is the potential danger to Mexico and other oil exporters and their creditors. The other is that a very sudden and swift decline in oil prices could be a destabilizing shock to the world economic and financial system.

Mexico, for example, loses about $500 million of export revenues a year for every $1 decline in the price of oil and it would need more cash from the United States and other nations if the oil price falls any further, Feldstein said yesterday.

Mexico has already agreed to an austerity program with the International Monetary Fund, in exchange for a loan which has formed the basis of a financial rescue package involving private banks and governments. This economic program "probably stretches Mexico's internal capacity to the limit," Feldstein said.