Lavish parties are rare events here, but the U.S. oil company Conoco decided to fete its arrival in this war-torn country on June 28 in unforgettable style.

It took over the newly renovated Panorama Hotel on an island across the bay here, hired two African bands, invited 400 of the capital's political and social elite and staged an all-night bash. Nearly the entire government showed up.

In this manner did Conoco, a subsidiary of E.I. du Pont de Nemours & Co. based in Wilmington, Del., "take the dive," as its resident manager, Jack Blackshear, put it, into the uncertain political and economic waters of Angola with a $60 million commitment to explore for offshore oil.

Increasingly, American firms are ignoring the public admonitions of the Reagan administration "to think about U.S. national interests" before coming here.

They are going forward, too, despite threats from U.S.-backed guerrilla leader Jonas Savimbi, who already has attacked the Angolan facilities of the oil giant Chevron.

One of the continuing paradoxes of this African bush war is America's conflicting political and economic investment here. While the Reagan administration is supporting Savimbi's guerrilla struggle by sending him sophisticated U.S. weapons and other covert aid, American oil industry titans are squarely on the other side of the struggle.

They are pumping the bulk of the oil -- 285,000 to 300,000 barrels a day -- that provides the Marxist central government with 90 percent of its foreign exchange, the wherewithal to stay in power and pursue its attempt to crush Savimbi's National Union for the Total Independence of Angola (UNITA).

This glaring contradiction in U.S. interests in Angola has not gone unnoticed by Savimbi or his conservative Republican backers in America, who have launched a campaign against Chevron and its Angolan subsidiary, Cabinda Gulf Oil Co., by pressing the Reagan administration to forbid U.S. business operations here.

Even as Conoco was busy setting up shop this summer in the coastal village of Ambriz, 60 miles to the north, Savimbi was spelling out -- in an interview with Washington Post reporter Patrick Tyler far to the south -- his restrictive terms for a live-and-let-live relationship with American oil firms.

"If they are making business as usual, we can't interfere," he said. "The French are making business, but they are keeping up their contacts with us." He suggested his forces would not "attack the French interests, if they don't make politics."Guerrilas Attacked Chevron Pipeline

But Savimbi had only belligerent words and sharp warnings for Angola's biggest producer here, Chevron, America's second largest international oil company, which he accused of lobbying against UNITA in the United States and of refusing to make contact with him.

Savimbi acknowledged in the interview that in early April -- only a month after he returned to Angola with his first shipment of U.S. military aid -- his guerrilla forces had attacked Chevron's Cabinda Gulf Oil.

Savimbi said the UNITA attack on an abandoned Chevron pipeline in northern Angola was a deliberate attempt to punish the company for statements one of its officers had made in Luanda in support of the government.

Savimbi also used the interview to warn that Chevron must stop making "big statements" in support of Luanda, that it must approach him privately ("We are not asking them to make any public statement," he said) and that it must give him assurances the company will not block UNITA efforts to win more U.S. aid.

In addition, he said, it must keep a low profile, as the other oil companies have.

But if Chevron continues "insulting us -- then we hit, then we hit. We say it is a wrong thing -- you shall not . . . do that!" he said.

In Luanda, however, where another reporter visited in July, the American general manager of Cabinda Gulf, Will M. Lewis, was not assuming a low profile. By funneling aid to Savimbi, he said, the United States is "backing the wrong guy here."

In an interview, Lewis advocated dumping Savimbi and urged Reagan administration officials to improve their relations with the Luanda goverment.

"If they would just establish diplomatic relations and get an embassy here," he said. "Don't they have diplomatic relations with Marxist Mozambique?"

Lewis also was critical of the administration's decision to freeze all loans and guarantees from the U.S. Export-Import Bank to American companies doing business here until Luanda resumes negotiations on a Cuban troop withdrawal from Angola and stops making war on UNITA.

The decision has forced Cabinda Gulf to turn to the French government for financing to develop a new offshore oil field capable of producing an additional 60,000 barrels a day for Angola, about a 20 percent increase.

Cabinda Gulf officials say it will take $160 million to $180 million to fully develop the new Numbi field, which lies about 12 miles offshore.

"We're just cutting the U.S. market out here," said Lewis, explaining that if the French export-import bank, known as COFACE, provided the financing, Cabinda Gulf would have to do business with French rather than American firms.

Cabinda Gulf, with a $1.3 billion investment here, has 80 existing contracts with other foreign, mostly American, companies for services needed to run its offshore wells, which are currently producing about 180,000 to 200,000 barrels a day.

"We're talking about a $200 million market here a year and we're cutting the United States out. French and British companies are taking the place of American companies.

"They are just raking it in because we can't deal with U.S. companies. They can't come up with the financing," he said.

Chevron has just completed negotiations to maintain a 49 percent interest in partnership with the Angolan government in two concession areas farther offshore than its present fields, which stretch 18 miles out from the Cabinda coast. It has committed itself to drilling 15 to 20 new wells, which cost about $6 million apiece, according to officials of the state-run oil company, Sonangol.

Chevron and Conoco are not the only well-known American oil firms doing business here. Texaco is the operator of one offshore field and has a stake in another. It has just invested $100 million more in exploration and the development of two new finds and has a commitment to drill at least four more wells, Sonangol officials said.Other Companies Show Interest

Tenneco and Arco have expressed interest in bidding on exploration rights for another block of offshore waters, according to Sonangol director general Herminio Escorcio.

Scores of American service companies and banks have a stake in the expanding oil industry here. Citibank and Bankers Trust both have helped finance the expansion and Arthur D. Little, the Boston-based consulting firm, provides advice to Sonangol.

Other industries also are seeking Angolan business. The Equator Bank, a Bahamian company headquartered in Nassau and with a U.S. office in Hartford, Conn., is negotiating with local authorities in Namibe Province, in far southwestern Angola, to set up a joint company that would export fish, marble and salt.

In the oil sector, the Angolan government has adopted an "open door" policy toward western companies despite its Marxist-Leninist ideology. It is not insisting on a majority share -- the stake it holds in Cabinda Gulf -- in new exploration and production ventures in the still largely unexplored coastal waters. Nearly 2 billion barrels of oil already have been found there.

The government is allowing groups of foreign companies to form their own joint ventures and inviting them to explore in various offshore "blocks" it has delineated all along the Angolan coast.

Conoco's Blackshear, a bearded, easygoing Texan, spent two years negotiating -- together with the Italian state oil company Agip and Spain's Espanoil -- an exploration and eventual production-sharing deal with the Angolan government. Conoco has agreed to drill six to 10 wells over a three-year period in a block that will cost $120 million to $150 million to be fully explored, according to Sonangol's Escorcio and his aides.

If the three western companies strike oil, Conoco will become the operator and the three western partners will have to hand over a certain percentage of daily production to the government.'A Good Dose of Pragmatism'

External Trade Minister Gaspar Martins said the Marxist government had no difficulty working so extensively with western firms. These firms are providing most of the $756 million scheduled to be invested in the expansion of Angola's oil sector this year.

"We're not dogmatic. We look at the systems of the world and decide what is applicable to Angola," he said in an interview. "There is a good dose of pragmatism in our actions to solve problems."

Martins defended Angola's preference for trade with the West, where it currently does 80 to 85 percent of all its business, despite the government's close political ties with the Soviet Union and Cuba:

"This is where the market is, where we sell and where we buy. We want technology from the West. There is no reason why this shouldn't continue."

One question being debated among western businessmen and Angolan officials is whether Savimbi really intends to follow a business-as-usual policy toward the oil industry if the companies do not lobby their governments to oppose aid to him.

His guerrillas this year have infiltrated heavily the Cabinda enclave, where Chevron is based, and moved into the Soyo district just below the Zaire River where many of the other oil companies, including Texaco, are working.

To help protect Soyo from attack, the Soviet Union has sent in more advisers to bolster Angolan Army units stationed there, and some reports say they are building their own base in that strategic corner of the country.

For now, Savimbi appears to be leaving his intentions toward American companies open.

"We want to reinforce our capability of making ambushes, of attacking in Cabinda and in the north because by doing so we diminish the pressure on the south," Savimbi said.

These ambushes, he added, will not be directed against American oil interests as long as the companies "are not involved in politics."

Savimbi then offered some advice to businessmen in Angola. "A very clever businessman," he said, "will leave all the doors open . . . and if the government changes, then you still have the door open for your business."

So to avoid "provoking the UNITA commanders in the area," Savimbi said, "Chevron should stop making those big statements."