As oil prices dropped below $16 a barrel in major markets today, a special committee meeting of the Organization of Petroleum Exporting Countries ended inconclusively, with oil ministers deciding OPEC must increase its production ceiling but not saying what the new ceiling should be.

Reflecting the uncertainty that now grips the once powerful oil cartel, OPEC President Arturo Hernandez Grisanti of Venezuela, who chaired the meeting of five OPEC members, said only that he would consult with the rest of the membership over the possibility of convening a gathering of the full 13-nation organization.

United Arab Emirates Oil Minister Mana Said Otaiba said a full OPEC meeting, technically not due until June, had been proposed for mid-March, but that no decision had been made.

On world oil markets today, prices for spot sales and futures prices for March delivery fell to the lowest point since 1979.

The March delivery price for the best-known U.S. crude oil, West Texas Intermediate, dropped to $15.44, nearly $2 below its close on Monday. British-produced Brent crude sold at $15.50 a barrel in Europe. This represented a 50 percent drop from late November, when comparable prices were about $31.

No such 50 percent price drop is expected for consumers, however, analysts cautioned. Gasoline and heating fuel prices to consumers are based largely on long-term contracts for oil purchases, which still reflect last fall's prices. While the drop in futures prices suggests a lowered cost to consumers eventually, oil company pricing policies could prevent that.

The question of where to set the new OPEC ceiling -- and the disagreement it has provoked within the organization -- appeared increasingly moot because leading OPEC producer Saudi Arabia already has more than doubled its output over the past several months.

While the 1981 self-imposed ceiling of 16 million barrels a day is still official OPEC policy, current daily production is estimated at 18.4 million barrels, with at least 5 million coming from the Saudis.

The Saudi objective is to flood the market and force non-OPEC producers such as Britain to lower output and share with OPEC the burden of dwindling oil demand. All OPEC members agreed to the strategy in principle in December, and appointed the special committee, composed of oil ministers from Venezuela, Kuwait, Iraq, Indonesia and the United Arab Emirates, to determine OPEC's "fair share" of the declining market and ways to defend it.

In the meantime, however, OPEC's less-solvent members already have felt the impact of the production increase on prices. Some, including hard-liners Libya, Iran and Algeria, have demanded that the strategy be dropped. But the Saudis, supported by other Persian Gulf producers, have indicated that they want to make the British give in, no matter what the cost.

Hernandez Grisanti said at a news conference that the decision of the five to raise the OPEC ceiling was "unanimous." The meeting here, he said, had "emphasized the absolute necessity" of the cooperation between OPEC and non-OPEC oil producing countries to achieve stability in the world market.

He acknowledged however that there had been no indication from Britain that it was prepared to cooperate.

Although Britain will suffer a substantial reduction in income as the result of the falling price of oil, Prime Minister Margaret Thatcher has said she will not interfere in production decisions of private oil companies operating in the North Sea.

She has said Britain can weather whatever financial difficulties might result and that in many ways lower prices will help its economy by lowering Britain's own energy costs.

The fluctuating anxiety of the past few weeks in London's money markets appeared to subside somewhat today as the pound exchange rate stabilized at around $1.37. At the same time, British Petroleum and Shell both announced they would lower their pump prices in Britain for a gallon of gasoline by about 5 to $2.50, in response to the falling price of crude oil.

Kuwaiti Oil Minister Ali Khalifa Sabah is scheduled to fly Wednesday to the Soviet Union, the world's largest oil producer, to discuss the situation, although Moscow has shown little interest in OPEC's dilemma.

Hernandez Grisanti said Mexico had been "the most supportive and collaborative of all non-OPEC countries." With 75 percent of its export revenue coming from oil, and a $96 billion foreign debt, Mexico stands to lose substantially if prices do not stabilize at a higher level.

Mexican Energy Minister Francisco Labostida Ochoa arrived here tonight to consult with Venezuela, its fellow Latin American debtor.