In the provincial capital of Cabinda south of here, the people still wear the long wrap-around skirts their ancestors displayed for the Portuguese five centuries ago, and the lush vegetation of the Congo River basin threatens to encroach on the closed shops and near-empty streets that pervade this struggling Marxist nation.

Travel the short way to Malongo, however, and you will find another world: air-conditioned, electrified, well supplied with hot water and Danish modern-style furniture, and filled with men who, after gorging themselves each night on food never dreamed of 20 miles down the road, retire to a bar for beer and popcorn.

This island of modern consumerism, so antithetical to a backward, East-oriented nation, represents Angola's most vital strategic interest: American-pumped oil. Here, in a cocoon that screens out politics as much as culture, Gulf Oil, like four other American oil companies, works hand-in-hand with a government the United States refuses to recognize, fueling an industry that accounts for some 70 percent of Angola's gross national product.

All the while, the Americans are benevolently protected by some of Angola's 15,000 Cuban troops -- forces that in Washington are called a prime threat to U.S. interests.

With $2 billion in annual oil revenues and hundreds of millions of dollars in U.S. investments at stake, business sense has long overshadowed politics in the far-northern Cabinda Province, separated from the rest of Angola by a 30-mile-wide strip of Zaire.

"The Cubans," said one Gulf employe as he watched the sun set over the gas flares from the off-shore oil wells, "protect the oil wells. That allows Gulf to lift the oil and Angola to sell it for dollars, which it uses to pay the Soviet Union for military equipment. The Russians then use the dollars to buy American grain."

No high oil official was willing to be as frank as this low-level employe. But officials of both the American companies and the Angolan government make it clear that the U.S. firms are here for business, that business is going well despite the Cubans and Russians, and that for now, they have no thought of limiting their operations.

"One of the buzzwords floating around the world is pragmatism," explained Tom King, Gulf's general manager in Cabinda. "I think it works here. . . . We are here on a business relationship. We are sensitive to the politics of the country but we don't interfere."

Indeed, Gulf, already the largest American investor in Angola, is planning to expand its interests in Cabinda oil. With current investments totalling $416 million, the company has just agreed to another $116 million, making its operations possibly the largest investment by an American company in a country not recognized by the United States.

Despite the Reagan administration's hostility to Angola, new American dollars are also coming from the U.S. Export-Import Bank, which recently approved a $85 million loan to Luanda for a gas-injection project designed to increase the production of off-shore wells. Morgan Guaranty Trust Co. heads a consortium of leading financial institutions that is arranging an accompanying $50 million loan.

Other American oil companies operating in or near the Cabinda area are Mobil, Texaco, Cities Service and Marathon. Six European and Brazilian companies are also active.

These American companies, traditionally cautious in foreign activities, have not attached themselves to the fate of the Angolan government. At Soyo on the southern bank of the Congo River, where Texaco has its operations, the superintendent of maintenance, Vic Bloot, noted that if South African-supported guerrillas of Jonas Savimbi's National Union for the Total Independence of Angola (UNITA) were to succeed in their war against the government, "We'd all have to leave" but "we'd be back in six months."

"No matter who is in power they will want to develop the oil," Bloot added.

For now, though, both the companies and the government profess to be satisfied with their relationship. Some of the American oilmen with experience in Nigeria, Africa's bastion of capitalism, even say they prefer to work in Angola because of better relations with the people.

In Luanda, Petroleum Minister Jorge Augusto de Morais said, "there is no contradiction at all in a Marxist country dealing with capitalist companies. We are independent. It is merely a commercial relationship."

In addition to the oil revenues, the Angolan regime has gained needed support in Washington from the companies, which have worked to block the Reagan administration's effort to allow U.S. military assistance to anti-government guerrillas. "The American oil companies are our best lawyers," said Angola's ambassador to France, Luis da Almeida.

Morais said the profits of the oil companies as a percentage of investment had increased under the current government, compared to the colonial reign of the Portuguese. The companies are allowed to take from 19 to 23 percent of cash flow as profit, he said.

Current total oil production in Angola, Morais said, ranges from 150,000 to 170,000 barrels a day, and is projected to increase to 200,000 next year. Of that, he said, Cabinda currently produces about 100,000 barrels a day and plans call for it to increase to 200,000.

Unlike colonial times, when oil companies purchased concessionary rights to explore for and exploit oil, the government has insisted on joint enterprises with the multinationals. The government oil company, Sonangol, has at least 51 percent ownership in all oil ventures in the country.

Gulf's King said the government participation, which began in January 1977, little more than a year after independence, "has not changed our operation. We have acquired a partner. We have a good rapport at the senior management level. This is not the first time we've done this kind of thing. This is a very agreeable government to work with."

Naturally, everything could not be quite as rosy as the self-serving remarks by both sides. The U.S. companies, despite their special status, have not avoided all of the economic and logistical problems afflicting the rest of Angola.

Supplies are sometimes difficult to obtain here -- it can take up to 18 months for goods to arrive. First both American and Angolan company officials must approve the order, which then is routed through the Ministry of Petroleum and the Central Bank, which has cracked down on imports. Meanwhile, there are the perpetual snags at Luanda's port, where 50 or more ships are usually backed up while waiting to unload. Currently, new roofs for the oil tanks at Soyo have been sitting on ships in the port for five months.

In addition, there are some personnel problems at Gulf because of the three-tiered personnel structure. Americans and persons of other nationalities employed by Gulf work a 28-days-on, 28-days-off schedule, and fly home when they are off. Portuguese workers under contract to Cabinda-Gulf work six weeks and then get four weeks off. Angolans, meanwhile, are generally on more regular work schedules.

Of the total work force of 625, about 350 are Angolans, 150 Portuguese and 125 are American Gulf employes, of whom about 100 are U.S. citizens.

For the self-styled "oil field trash" who commute monthly between the United States and Cabinda, Angola is known only through Gulf's secure work camp in Malongo and the 350 Angolans who work there. "It's the same as being in prison -- except we're fed better," said American "Tennessee" Utley.

Elsewhere in Angola, the facilities at Malongo base would hardly be regarded as a prison. In addition to the hot water, air-conditioning, and comfortable rooms, the cafeteria, which serves more Portuguese and Angolans than Americans, is full of products familiar in the United States but unheard of in a Marxist African country -- ice cream, fruit juices, coffee from dispenser machines, and even French's mustard.

And yet, Americans still accustomed to living in their own country, find they are limited to four cans of beer a night, no hard liquor, no women and no travel outside the base.

"This is not a place where it's desirable to have a lot of discretionary time, because there's not a lot to do," said King.

The work regimen for the Americans and the third-country nationals calls for a 12-hour workday six days a week and a "half-day" on Sunday of about eight hours. The men mainly work in supervisory positions, so are usually on call at night.

The benefit comes after four weeks, when the men get four weeks off and receive a paid trip to their homes, mostly in Texas, Oklahoma and Louisiana. Gulf in effect has two full crews of people, one on and one off, at any given time, as do other oil companies working in Angola. Most of the air flights to and from Europe are filled with commuting oil workers.

Many of the men say they are working in Cabinda mainly because of the lucrative time off.

"I have 28 days to do what I want," said Bob West, who lives with his wife and three children in east Texas when he is not at Cabinda.

"It's my time," he added, saying he mainly fishes and hunts in his time off after finishing "honey-do lists." He explained that on his return after a four-week absence he is always confronted by his wife saying, "'Honey, do this' and 'Honey do that.'"

"I've been married 28 years, my wife's been married 14," West said in reference to his monthly commuting schedule, which he has maintained since 1967 in a variety of oil countries.

Utley, 64, has been working in the oil fields for 40 years, most of it overseas. He retired but then "un-retired" about a year ago. His wife lives in Fort Smith, Ark.

The camp has limited recreational facilities -- pool tables, a pocket-book library and a nine-hole golf course with dirt "greens." The main distinguishing feature of the course is a huge flaring gas jet.

The men declined to say how much bonus money they get for overseas work, but said it speeds their pension eligibility. At Petroangol, a combined Belgian-Angolan firm working at Soyo, operations manager Jim Swatten said the overseas employes are paid a bonus of about 80 percent, but these men work six weeks before each four-week break.

The Portuguese contract workers at Gulf are on a similar schedule, while the Angolans working on the oil platforms work two weeks and then get a week off.

One such worker, Ambrosio Alexander, 39, who works on platform Charlie, about three miles out in the Atlantic Ocean, spends two weeks on a 6 a.m. to 6 p.m. shift, monitoring Charlie's equipment, then has a week off and switches to the night shift. He makes about $400 a month, about twice the average Angolan salary, although about a third of it is in overtime.

There is little racial mixing at Cabinda-Gulf, largely because of the vast differences between the local work force and the one that commutes monthly across the Atlantic.

Presently, Angolans do not hold higher-level positions because of the acute shortage of trained persons.

"We will take any university graduate we can get hold of," King said. "It is difficult to come by graduate staff." The country's university only has about 2,000 students.

Gulf and Sonangol are creating an Angolan training program designed to meet a government target of 75 percent Angolan staff by the end of the decade.

In large part, the American-Angolan ventures have survived the transition from colonialism because the various liberation groups who fought the independence struggle and the wars that have followed it left the oil facilities alone.

That secure status continues for the most part today, although recently a mine was found along the oil pipeline, apparently planted by a group called the Front for the Liberation of the Cabinda Enclave (FLEC), Gulf official Jose Drummond acknowledged.

This guerrilla band carries out sporadic armed attacks in Cabinda in an effort to gain independence for the province, which has a 60-mile coastline and extends about 75 miles into Zaire.

Cuban troop strength in the province was beefed up after a 1977 coup attempt in Luanda. Now, a sign at the entrance to the Cabinda town stresses the political importance of the enclave. It says, "Angola -- One people, one nation, from Cabinda to Cunene," the southernmost province.

But King, who like other Gulf employes in the Malongo sanctuary, says he is hardly aware of the turmoil in the rest of Angola. As for the Cuban troops, which are the focus of so much antagonism in Washington, he says, "I hardly ever see them. I've never dealt with them."