A renewed warning of an oil price war by Saudi Arabia's energy minister helped keep oil prices sliding downward yesterday, for the sixth day in a row, to well below $20 a barrel.

Once again, the trading in London and on the New York Mercantile Exchange was volatile and heavy. The selling began yesterday in Europe after Ahmed Zaki Yamani, the Saudi oil minister, warned of an unrestrained price war unless Britain and other producers outside the Organization of Petroleum Exporting Countries agree with OPEC to limit oil production.

Without an agreement, "There will be no limitation to the downward price spiral, which may bring crude prices to less than $15 a barrel, with adverse and dangerous consequences for the whole world economy," Yamani said, according to OPEC's official news agency.

A month ago, $20 a barrel represented the low end of most analysts' forecasts. But yesterday prices of Brent North Sea crude oil, the benchmark oil on European commodity markets, plunged to $17.70 a barrel in early trading before closing at about $18.30 a barrel, compared with Wednesday's price of $19.90 a barrel. The prices are for contracts to buy or sell oil for delivery in February.

The price of West Texas Intermediate crude oil, the most widely traded U.S. crude oil on the New York exchange, dropped to $18.75 a barrel in an opening selling surge triggered by Yamani's warning. However, the price pulled up to $19.77 after speculators, major oil companies and other traders in the industry began buying. That price was down 52 cents from Wednesday's closing price.

Yamani's warning appeared to be aimed at Britain and Norway, the main targets of OPEC's pressures. The Saudis want the two North Sea oil producers to take a major share of the reduced demand for crude oil when the heating season ends this spring.

But Britain's Department of Energy said yesterday it has no intention of curbing North Sea oil production. "There is no change in our policy to leave control of production in the hands of the companies," a department spokesman said. Norway's oil minister said this week his country would restrain production only as part of a general agreement by OPEC, Britain and other non-OPEC countries to stabilize production. Yamani was quoted as praising Norway for a "pragmatic and realistic approach," but he criticized Britain.

Mehdi Varzi, an oil analyst at Grieveson, Grant & Co. in London, predicted that continued production by Saudi Arabia "will bring the oil price so low that the U.K. cracks," the International Herald Tribune reported.

Meanwhile, concern about the impact of lower oil prices continued to focus on Mexico, which depends on oil revenue to carry a $97 billion foreign debt load, and on banks in the American Southwest with large loans outstanding to the domestic energy industry.

Assistant Secretary of the Treasury David C. Mulford told a congressional hearing yesterday that, if oil prices stabilize at $20 a barrel, Mexico's problems are "containable." It would face a trade deficit of $4 billion in 1986, based largely on the drop in the value of oil exports. That figure is about what Mexico was expected to seek in new borrowing this year from foreign banks and international agencies before oil prices dropped.

If oil prices decline sharply below $20, however, Mexico's new borrowing needs probably would rise quickly, from $4 billion to as much as $6 billion or $6.5 billion, Mulford said in testimony before the Joint Economic Committee.

Thus far, the sharp drop in oil prices on commodity markets is not indicative of actual prices that producing nations such as Mexico are receiving for their oil, Mulford said. The fall of prices for the Brent and West Texas grades "doesn't mean the whole oil market has shifted," he said. "It is disturbing," though, because, unless reversed, the trend in commodity prices will be followed by declines across the board. The abrupt fall in prices has forced Texas banks to rapidly recalculate the risks they face because of extensive loans to the depressed U.S. energy industry, said William Gibson, a senior vice president at RepublicBank Corp. in Dallas. His bank, the largest in the Southwest, has reduced its energy loans to 12 percent of its total portfolio from 20 percent earlier in the 1980s. The same strategy has been followed at Texas banks in general, where the average proportion of energy loans was even higher, he said.

At $20 a barrel for oil, the problem is "manageable," he said. "I hear Texas bankers say, 'Down to $20 we'll be all right. Below $20, it's just the unknown.' " That is an unknown he doesn't expect to encounter, he said.

Amoco Corp., the nation's fifth-largest oil company and one of the first to report financial results for the final three months of 1985, said yesterday that its quarterly profits declined to $390 million ($1.51 a share). That was a 16 percent drop from a $465 million profit ($1.72 a share) for the last three months of 1984. Profits for the full year were 11 percent below the 1984 level. The company made record profits on energy refining, marketing and transportation, but those gains were more than offset by lower crude-oil and natural-gas prices, higher exploration costs and the impact of foreign currency changes, the company said.

Sohio, the 12th largest oil company, said it lost $771 million in the fourth quarter last year, chiefly because of a modernization and reorganization of its copper operations in Utah. It had a profit of $290 million ($1.25 a share) for the last three months of 1984.