The U.S. merchandise trade deficit dropped to $12.1 billion in April, its lowest level since August, because of falling imports of oil and manufactured products, the Commerce Department reported yesterday.

But Commerce Secretary Malcolm Baldridge warned that last month's improved figures "probably do not mark a turning point" in more than four years of record deficits that have spawned increased congressional calls for a more aggressive trade policy by the Reagan administration.

"I expect the trend in the deficit to begin to improve later in the year," Baldrige said.

Administration officials have said they expect this year's deficit to be about the same as last year's record $148.5 billion. The trade figures over the next few months are critical to the Reagan administration campaign to derail congressional efforts to toughen trade laws and to maintain the support of recalcitrant Republican legislators, who are fearful of losing House seats and control of the Senate over the trade issue.

The House last week passed a wide-ranging trade bill that President Reagan attacked Thursday as "kamikaze legislation" that will cost millions of American jobs and "send our economy into the steepest nose dive since the Great Depression." The Democratic-sponsored bill also gained the support of 59 Republicans.

The White House, moreover, is concerned that the Senate will produce a version almost as repugnant to Reagan as the House bill.

Laying down a line he hopes Republican lawmakers will pick up in their November election campaigns, Reagan argued in a speech to the National Association of Manufacturers Thursday that his administration's free-trade policy -- a combination of selected attacks on unfair trade practices and moves that have lowered the value of the dollar by more than 30 percent -- will improve the trade picture.

April's decline in the trade deficit was helped by a drop in the value of oil imports to $1.8 billion, their lowest level in 11 years, as the average price per barrel plummeted to $14.93. This is a drop of $10.70 from the average price of the previous five months, bringing oil prices for April to their lowest level since March 1979.

But Baldrige indicated that the monthly trade figures would get little help in the future from further drops in oil prices. "The declines in oil imports are largely over," he said.

Imports of manufactured goods, a key element of the U.S. economy, also dropped by $2.2 billion from March, to $23.1 billion. But overseas sales of U.S. manufactured goods fell from $13.3 billion in March to $12.6 billion last month.

"The pickup in economic growth in the United States will have to be matched by a more rapid expansion abroad if U.S. exports are to increase significantly," Baldrige said.

Jerry Jasinowski, chief economist and executive vice president of the NAM, was pessimistic about the April trade figures. He said they showed "no improvement in manufacturing, which accounts for 76 percent of U.S. trade activity.

"The dollar is having some effect in slowing imports," he said, "but it has had no significant impact in raising exports."

He pointed out that the U.S. trade deficit for this year would be $167 billion, based on the figures for the first four months.

"In manufacturing, the story is much worse, with the deficit on an annual rate running at $134 billion," Jasinowski said, compared with $112.8 billion last year. "Exports are up only 2 percent, the same rate of increase as last year, while imports are up 8 percent. This works out to a worsening in our manufacturing deficit of $21 billion at an annual rate."

The April figures showed, for instance, that auto imports, especially $1.5 billion worth from Japan, increased by 11.6 percent.

Steel imports continued their decline, which Erwin L. Klein, president of the American Institute for Imported Steel, said was because of restrictions on foreign steel sales ordered 19 months ago by Reagan. Thomas C. Graham, chairman of the American Iron and Steel Institute, called the decline "a step in the right direction," but said the goal of the president's program still has not been achieved.

Economists were divided on the meaning of the April figures. Alan Sinai, the chief economist for Shearson Lehman Bros., said, "The worst of the trade deficit is over." But Michael Evans of the Washington forecasting service Evans Economics said, "It looks good, but it isn't. The fact that exports went down is not a good sign."

While the U.S. deficit with Japan remained the largest, it declined by $800 million in a month, to an April level of $4.7 billion. The deficit with Western Europe stood at $2.7 billion, about the same as March, while the deficit with Canada was $1.8 billion, down $500 million, and the $1 billion deficit with Taiwan was down $200 million.