An unusually sharp increase in oil imports pushed the nation's foreign trade deficit to a record $5.6 billion last month, intensifying the worsening that began in December, the government reported yesterday.

The large jump in petroleum purchased reflected both higher prices and increased volume. The value of U.S. petroleum imports rose $7.7 billion, or 18.2 percent, while volume jumped 9.2 percent to 256.8 million barrels.

The new surge in oil purchases sent the nation's overall import bill soaring 3.2 percent, following a 6.2 percent boost in January, while export levels dipped 0.7 percent after rising 3.6 percent in January.

The overall deficit was the largest since a $5.2 billion red-ink figure recorded in February 1978. The trade deficit had been shrinking during the last half of 1979, but began widening again in December.

As they were the previous month, the trade figures were calculated under a new congressional formula that requires officials to include the cost of insurance and freight -- a sharp departure from traditional procedures.

The new formula was mandated by Senate conservatives who frequently have sought support for tougher import restrictions.Private analysts say the new calculations produce a trade deficit about $1.2 billion larger than the old.

Under the law, the Commerce Department is required to wait 48 hours before publishing the traditional calculations, which are expected to show a red-ink figure of about $4.4 billion rather than the $5.6 billion reported yesterday.

Economist traditionally have favored the previous method of calculating the trade figures, on grounds that insurance and freight charges are added services the importer buys and do not measure the cost of the product.

The increase in oil imports last month in large month in large part reflects the impact of crude-oil price-hikes announced in December by the oil-exporters' cartel. Spotmarket prices also rose sharply in January and February.