Jonas Savimbi, the United States' longtime and close ally in the Angolan civil war, has presented policy-makers with a problem. He says that he intends to blow up U.S. oil firms operating in Angola.

At a news conference at the Carnegie Endowment for International Peace here yesterday, the 51-year-old guerrilla leader confirmed his movement's intention to attack the Angolan facilities of Chevron/Gulf, but said exactly when is a question of "strategy" and "timing."

Savimbi has openly criticized Chevron/Gulf and other U.S. oil firms for providing the Marxist government of Angola with the $480 million that he says the government pays annually to Cuba for its estimated 35,000 troops and advisers in the African nation. Last May, Savimbi tried, with South African help, but failed to blow up Chevron/Gulf's installations.

Later yesterday at a National Press Club luncheon, he said it was "morally wrong" and "unacceptable" for Chevron/Gulf to be providing money to the Soviets and Cubans for their backing of the Angolan government. "We think that we must stop Gulf," he said. But he later added, "I'm giving the assurance that American lives will not be endangered."

Chevron/Gulf has 687 employes in Angola, 150 of them Americans.

Savimbi, backed strongly by South Africa, has been pushed by the Reagan administration and the American conservative establishment as a model guerrilla figure in their campaign to promote anticommunist freedom fighters around the world. Savimbi asserts that he intends to show he can use "the guerrilla tactics of Mao and Fidel Castro successfully against the Soviets."

The bearded guerrilla leader arrived here Tuesday on a 10-day lobbying campaign for U.S. diplomatic and material support for his struggle.

As in 1975-76 when the Central Intelligence Agency aided him and other pro-western forces with arms and training, Congress, the administration and the public are again divided over the wisdom of an U.S. involvement in Angola.

The debate is taking place against the background of a predicted military showdown this spring between the Angolan government and Savimbi's National Union for the Total Independence of Angola (UNITA).

Caught in the middle of what may become a major shift in U.S. policy toward Angola and all of Africa are U.S. oil companies, particuarly Chevron/Gulf, the largest producer in Angola and chief source of Angola's $2 billion in annual oil revenues. Texaco also has an interest there, and another U.S. oil firm, Conoco, is negotiating with the Angolan government to begin off-shore exploration.

Chevron/Gulf alone, with a $600 million investment in its Angolan installations, paid the central government almost $600 million in taxes and royalties in 1985, according to a company spokesman. Its 49 percent share of production in the Cabinda offshore field amounts to 80,000 barrels a day, half of which is shipped to the United States.

Apparently aware of the squeeze between U.S. policy and U.S. economic interests, the administration is hinting at a major shift in its position on American business involvement in Angola. For a decade, it actively supported and encouraged U.S. firms to invest there, providing $214 million in Export-Import loans and guarantees partly to Chevron/Gulf and partly to the Angolan government despite the lack of U.S.-Angolan diplomatic relations.

However, in a pointed warning Tuesday to U.S. firms there, Assistant Secretary of State Chester A. Crocker said the administration did not intend to say "through our export policies that there's any support of ours for the Angolan war effort." He advised the companies to start think about "U.S. national interests" as well as their own corporate ones.

Crocker's warning caught Chevron/Gulf by surprise. Chevron spokesman Stephen North said Wednesday the State Department had never communicated any change of policy to company officials, and "it's really very confusing."

Yesterday, another Chevron/Gulf spokesman, G. Michael Marcy, said company representatives had been told by State Department officials -- on the day Crocker was issuing his warning -- that they regarded Chevron/Gulf's presence in Angola as "important" and hoped it could play "a helpful" role in achieving a settlement of disputes between Angola and South Africa.

Chevron/Gulf -- formed when Chevron acquired Gulf and its Angola operation last year -- is in the process of using a $96 million loan guarantee from the Export-Import Bank for expansion of its activities in Angola and has placed contracts with 22 U.S. firms for services.

A senior administration official said Thursday there are "probably close to 100 U.S. firms" doing business with the Angolan government.

The Export-Import Bank, a U.S. Government institution, also seemed caught off guard by Crocker's warning. The bank is scheduled to disperse $50 million in loans to the Angolan government and its state oil company, Sonangol.