A record drop in oil imports last month helped cut the nation's trade deficit to its lowest level since 1975, the Commerce Department reported yesterday.

The $451 million gap between imports and exports last month compared with one of $3.15 billion in February and $5.4 billion in January, the report said. The United States traditionally runs a deficit on merchandise trade that is offset to some extent by a surplus in the trade of services, such as engineering or insurance and income from overseas investment.

Exports climbed by 8 1/2 percent in March to $21,434 million. At the same time, total imports dropped by 4 1/2 percent to $21,885 million, with plunging oil imports more than offsetting a rise in non-oil imports.

In March only 5.5 million barrels of oil a day were imported, while in February the figure was closer to 7.5 million. Total oil imports in the two months were 169 million barrels last month and 210 million barrels in February.

Commerce Deputy Secretary Joseph Wright called the news "highly encouraging." He commented "oil imports during March were the lowest in more than five years and the outlook for the coming months appears to be favorable."

The balance of payments was surprisingly strong in the first three months of the year, according to gross national product figures published last week by the Commerce Department. These contributed to the vigorous 6 1/2 percent annual growth recorded in the preliminary figures for the first quarter.

Yesterday's figures suggest that the trade performance was even better than the first estimates for the January-to-March period, Commerce economist William Cox said yesterday.

Exports between January and March averaged $240.1 billion at an annual rate, about 9 percent higher than the 1980 level. Imports in the same period were running at an annual rate of $276.2 billion, 7 percent up on the 1980 level.

Despite the improvement in trade last month, non-oil imports continued to rise. Auto imports rose for both the month of March and the first quarter, Wright noted. The trade balance in manufactures nevertheless improved sharply in both February and March.

A deficit of just under $500 million in the manufacturing sector in January reversed to a surplus of $1.6 billion in February and $1.88 billion last month. The trade picture is somewhat surprising given the continued strong growth in the economy, experts say.

The dollar recently has been strong against the currencies of most other industrialized countries. The good trade figures could encourage that strength, an analyst said.

Wright warned that "the extremely low deficit in March may have been due to a confluence of favorable factors which may not be repeated." However two political factors -- the lifting of the U.S. embargo of grain to the Soviet Union and the "possibility of reduced Japanese auto imports to the U.S., may be positive factors in the trade picture for the month ahead," he said.

The U.S. trade deficit with the oil-exporting nations shrank by $1.4 billion between February and March under the impact of the reduced oil imports. It totaled $2.1 billion last month.

Western Europe stepped up its purchase of U.S. goods between February and March, the Commerce figures showed. There was an improvement in the trade surplus which the U.S. has run with these countries from $1.5 billion February to $1.8 billion last month.

Wright said the administration plans "to redouble our efforts to increase export consciousness among American businesses and to revitalize the international competitiveness of our industry."