Opposition newspapers and politicians are accusing the South African government of misusing a security law to cover up the story of how it bought $50 million worth of stolen oil from a syndicate of international crooks, and then paid another $30.5 million to the oil's rightful owners after the con was exposed.

Until a few weeks ago, there was no reason for the public here to make a connection between their government and the three-year-old tale of a scuttled 200,000-ton supertanker, the Salem, that sank off the coast of West Africa in early 1980. Although the scuttling, which launched an investigation and a series of insurance claims in Britain, was widely publicized elsewhere, the story was largely unknown in South Africa.

On Feb. 16, however, a member of the opposition Progressive Federal Party, John N. Malcomess, made a speech in the House of Assembly in which he set out the facts of the situation and accused the government of misusing the security law to cover up a scandal.

The story provides a revealing glimpse not only into the power of the government to suppress damaging news, but also of a field of international intrigue into which South Africa entered, perhaps only through what Malcomess called "mere gullibility," in order to breach an Arab embargo against oil shipments here.

The gist of the tale, obtained by Malcomess from British court documents, is that South Africa helped the conspirators buy the Salem to ship the oil from Kuwait to Durban. The conspirators, according to the documents, pocketed the $50 million fee for the oil, scuttled the ship and, when an insurance investigation was launched, left South Africa responsible for another $30.5 million in compensation to the rightful owners of the oil, Shell International.

Shortly after the Salem sunk off West Africa on Jan. 17, 1980, the government invoked the Petroleum Products Act, a law passed when the Arab states imposed their embargo in 1977. Its purpose was to protect the secrecy of South Africa's oil procurement deals and it prescribed a penalty of seven years' imprisonment for publishing any unauthorized information about oil. Newspapers here were warned again of the law when insurance cases on the Salem were heard in Britain a year ago.

When Malcomess made his speech two weeks ago, newspapers pounced on the opportunity to run the Salem story because parliamentary privilege, which protects legislators against prosecution or libel actions, extends to press reports of what they say in the House.

But Energy Minister Pieter T. Du Plessis attempted to reimpose his gag rule, prohibiting the country's largest circulation paper, The Sunday Times, from publishing what Malcomess said. Two other papers, however, The Sunday Tribune and Sunday Express, defied his order and published.

Speaker Johan Greef ruled the ban unconstitutional and Du Plessis backed down, amending it to an appeal to "take care that what we say does not place the people who provide us with oil in an embarrassing position."

The facts the South African government has been at such pains to conceal from its public, and which have now been exposed by Malcomess, reveal grand-scale resourcefulness on the part of the conspirators and what Malcomess has suggested was gullibility on the part of South African officials.

It began with the fall of the shah of Iran, whose country until then furnished all of South Africa's oil.

Realizing South Africa would be hard pressed for supplies because of the Arab embargo, the conspirators--not named by Malcomess--got in touch with representatives of the country's Special Fuel Fund Association, whose task it is to procure supplies and which is connected to the large state-run oil-from-coal company called Sasol.

They offered to ship four supertanker cargoes of oil to South Africa, but said they would need South Africa's help to buy the tanker.

South Africa accepted. The fund's officials got a local bank, Mercabank Ltd., to issue to the conspirators an irrevocable letter of credit for $12.3 million, to be deducted from the price of the first shipment of oil.

With this the conspirators bought a 200,000-ton tanker, South Sun, without paying a cent for it themselves.

Their next step was to form three phony companies to serve as covers for their operation. One was Beets Trading AG, with an address in Zug, Switzerland. Another was a Liberian company called the Oxford Shipping Co. Inc., which had never traded. They changed the name of the South Sun to Salem and registered it with Oxford.

Then, with consummate daring, the conspirators offered their ship for charter to carry a cargo of oil from the Persian Gulf to Europe. A reputable Swiss company, Pontoil S. A. of Lausanne, took up the offer and asked the Salem's owners to load 200,000 tons of Kuwaiti crude for Italy.

The conspirators took on a new crew, said in the British court papers to have been made up of Greek officers and 22 Tunisian seamen, all of whom were in on the plot.

The Salem put in to the Kuwaiti port of Mina on Dec. 5, 1979 and took on 195,000 tons of crude, for which Pontoil paid the Kuwait Oil Co. The ship headed south to round the cape, ostensibly for Italy. Meanwhile, Pontoil sold the cargo to Shell International Petroleum Co. Ltd.

At about the same time, the Salem underwent another name change. The conspiratorial crew painted out the "Sa" at the beginning of the name and added an "a" at the end, making it the Lema. This apparently was intended to blur its identity because another tanker, the Lima, also with a cargo for Shell, was sailing a day or two behind.

On Dec. 27 the Lema, as it was then called, turned off its course for Europe and put into the South African port of Durban where, the next day, it discharged 180,000 tons of oil. A pump fault prevented the discharge of the remaining 15,000 tons.

South Africa paid for the oil, $12.3 million going to the sellers of the tanker. The rest went to Beets Trading AG. It was remitted by telex and within minutes was distributed among the conspirators through numbered Swiss bank accounts.

The Lema sailed out of Durban, refilled its tanks with sea water and continued northward, looking to all the world as though it still carried a full cargo of oil.

Off the coast of Senegal, in the deepest part of the Atlantic, there were a series of explosions on board and the Lema began to sink.

Conveniently close by was another tanker, the British Trident, which picked up the Lema's crew. The captain of the British Trident took a movie of the sinking tanker. It showed that the ostensibly laden supertanker left only a tiny oil slick as it sank.

That is what started the insurance investigation that eventually elicited the facts featured in the British cases. It is from there that Malcomess culled them to tell the story for the first time to the South African taxpayers whose money it was that sank like the Salem.

According to the insurance case records, a preliminary inquiry was held in Senegal. The captain, a Greek, produced his credentials which turned out to be a forged Liberian master's certificate. He and the first officer were extradited to Liberia, but were released following a coup in that country. The main conspirators behind the whole fraud have never been traced.