In a move that appears to herald the beginning of a new economic policy, Angola's Cuban-backed Marxist government is turning to Western companies to expand oil production in this mineral-rich county.

Western oil sources say the government has just published a basic law regulating foreign participation in expansion of what they regard as promising and still largerly unexplored areas onshore and along the Angolan coast. The law provides for Angolan state ownership of all newly found oil deposit, but also for sharing production between the Angolan state oil concern, SONANGOL, and the foreign company that makes the discovery.

The U.S. goverment has not recognized the Angolan government under the Popular Movement for the Liberation of Angola, the faction that defeated two American-backed nationalist groups during the 1975-76 civil war, with Soviet and Cuban assistance. But this has not prevented President Agostinho Neto and his lieutenants from dealing with U.S. companies.

"They are very pragmatic in their approach and encouraging Western companies to come in," said one source familiar with the new oil law, which reportly was drawn up with the help of American consultants.

He said the Angolan government's main interest is encouraging foreign companies to explore and drill as quickly as possible to boost production rapidly.

Among companies showing strong interest in Angolan fields are four American ones-Exxon, Standard Oil of California (Chevron), Phillips Petroleum Co. and Cities Services Co.-in addition to a number of European firms, including Shell, British Petroleum and Total.

Neto told a group of American reporters accompaning Sen. George McGovern (D-S.D.) on a vist here last week that Angola has had "very good relations" with American companies.

In private talks, Neto reportedly made a strong pitch for more American investments in his economically depressed nation.

The main oil company already in Angola is American, Gulf. Its daily production is about 135,000 barrels from a filed off the coast of Cabinda, a tiny enclave north of the Zaire River that belongs to Angola but is surrounded by Zaire.

Recently, Cuban troops were deployed around Gulf storage facilities to protect them from antigovernment guerrillas. The threat has now subsided, according to sources here, although Cuban Troops still are stationed in Cabinda.

Gulf alone provides the Angolan government with annual revenues of between $500 million and $700 million and is by far the most important source of foreign exchange. According to some estimates here, it accounts for 80 percent of the total.

Gulf has just signed a new agreement giving the government a majority interest in its Angolan operations and it is reportedly ready to make a new multimillion-dollar investment to increase production at its offshore Cabinda field.

Another American company, Texaco, has long been present with an interest in a small field in northern Angola near Santo Antonio do Zaire and a nearby offshore deposit that has yet to be developed.

The Anolan government also has signed on the Boston-based consulting firm Arthur D. Little to advise it on oil legislation. The company has provided advice to the Algerian government, which is understood here to have had some influence on Angolan thinking in oil matters.

Western oil sources say that under the new law a foreign company can expect 25 to 30 percent of any production resulting from a new discovery for which it is responsible.

Such production sharing is by now fairly standard in Africa and the Middle East but Angola is giving a premium in its terms to companies that put down the largest number of exploratory wells and get production going the fastest, according to these sources.

Asked how promising Angola is compared to other oil-producing West African countries, one source noted there had been a six-year moratorium on exploration here and added: "only drilling will tell."