Energy Secretary James R. Schlesinger Jr. warned yesterday that President Carter may slap a $5 a barrel import fee on foreign oil if Congress doesn't approve his plan to raise taxes on domestic oil.

"We are in this ludicrous position in which we are subsidizing every barrel of oil that comes into this country." Schlesinger said on "Issues and Answers" (ABC, WJLA). "That is the reason the equalization tax was proposed. If Congress fails to enact that tax, the President must consider other measures."

Schlesinger said the United States "cannot afford" to import oil in the amounts it now it, more than 8 million barrels a day and roughly half the oil burned in the United States. He said the United States will pay out $45 billion this year for imported oil.

The $4-a-barrel tariff was mentioned by Schlesinger because that is the fee that would have to be levied to reach the same goals as the equalization tax, which would raise domestic prices by about $2.50 a barrel.

The goal of either a domestic tax or a foreign tariff is to raise gasoline prices by 5 to 7 cents a gallon in an effort to discourage gasoline consumption, which is now at an all-time high.

The crude oil equalization tax would price U.S. oil at the same level as world oil, now about $14 a barrel. It would raise almost $15 billion in revente, which Carter has proposed to rebate to consumers.

The house approved the equalization tax, but the Senate Financa Committee voted it down last week in a disagreement as to how to spend the revenue. Sen. Henry M. Jackson, (D-Wash.), chairman of the Energy Committee predicted it will run into rough seas if it is brought up on the senate floor.

"The tax is not so much the problem as it is how it will be spent," Jackson said on "Meet The Press" (NBC, WRC). "Anytime you get $12 to $15 billion in pot before the Senate, [TEXT OMITTED FROM SOURCE] as to how it should be spent."

Schlesinger said one reason to put a tax on oil imports would be to get industries to switch to coal from the oil they burn.

"It is an essential ingredient in our whole coal conversion program, which is intended to save us 2 million barrels of oil a day," Schlesinger said. "Unless we are able to make the price of coal attractive relative to oil, we are not going to have that movement towards coal . . ."

Sources said Schlesinger discussed with aides last week the legality of an oil import tax as soon as the Senate Finance Committee voted down the crude oil equalization tax. The sources said Schlesinger presented the plan to Carter when aides assured him an import fee would be legal.

The legality of an import tax comes from a Supreme Court decision two years ago in which the court ruled that President Ford had the constitutional authority to impose a $1-a-barrel tariff on imported oil. The court ruled Ford was granted the authority under the Trade Expansion Act of 1962, which was amended in 1974.

Schlesinger was asked how the Arab countries might react to an import tax. He answered: "I think the oil exporting nations fully appreciate that the U.S. cannot go on subsidizing imports." CAPTION: Picture, JAMES R. SCHLESINGER JR. . . . warns of $5-a-barrel levy