An Organization of Petroleum Exporting Countries planning committee agreed here today on the principle of gradually increasing oil prices at three-month intervals as part of a long-range economic strategy for the world's major oil producers.

The proposed plan calls for crude oil prices to be adjusted quarterly on the basis of the inflation rates, currency values and economic growth of Western oil-consuming countries.

Venezuela's oil minister, Humberto Calderon Berti, said this method would likely raise oil prices each year by 2 to 3 percent on top of the Western countries average inflation rate.

But Calderon, OPEC's current chairman, admitted at a press conference yesterday that this change could not be implemented until OPEC countries again agreed on a unified pricing system. This system broke down during December's OPEC price meeting in Caracas. "Official" prices now vary by as much as $11 -- from Saudi Arabia's $26 a barrel to the $37.21 charged by Algeria.

Calderon said he thought the 13 OPEC nations could return to a unified price by their next price meeting in Algiers in July. But other oil ministers at the Tuesday meeting here and oil industry analysts were more pessimistic.

"The days of an official market price have passed," said a top industry analyst here. "OPEC's role has been overtaken by events. We now have a more classic market situation. Individual countries will continue to charge whatever they can get."

Crude prices have continued to rise steaily. Analysts here note that while some oil, such as Algeria's, is selling at top price, other countries, including Nigeria, have had difficulties signing up new contract customers at the most expensive prices. Analysts also say prices on the noncontract "spot market," where about 15 percent of the world's oil is now sold, are showing signs of leveling off because the present world supply slightly exceeds the Western countries' somewhat lower demand for oil.

Calderon told reporters that the OPEC committee's long-range strategy proposal anticipates that the OPEC countries' oil production would remain around the present level of 30 to 32 million barrels a day over the next 10 years, although the differences in production policies among OPEC members are also appearing.

Kuwait and Venezuela intend to cut back production to bring the world supply down to present demand. Kuwait and other Persian Gulf producers with small populations and plenty of cash on hand also are under domestic pressure to hold down production and conserve their reserves for the future, while more populous Algeria and Nigeria want to sell as much oil as they can to finance their developing economies.

Calderon did not discuss the other significant new trends among OPEC nations: selling less of their crude to major oil companies and insisting that the companies cut the producers in on a share of processing and marketing profits in return for continued supplies of crude oil.

The major oil companies -- Exxon, Shell, British Petroleum, Mobil, Texaco Gulf and Standard of California -- now handle only about 40 percent of the oil produced in the noncommunist world, as opposed to 60 percent just 10 years ago. Much of the rest is going to government-owned oil companies in nation-to-nation deals, in which the oil-producing nations sometimes seek trade concessions besides cash from the consuming countries.

The oil producers also are seeking and getting extra considerations from the major oil companies when they do sell them crude. New contracts for Saudi crude, for example, provide for British Petroleum and Mobil to process some of the oil for Saudia Arabia's government-owned Petromin oil company.Iran's new contracts to supply British Petroleum and Shell require them to share their processing and marketing profits with the National Iranian Oil Co.

Kuwait, which is combining its oil companies into a new Kuwait Petroleum Corp., is expected to seek similar concessions when it renegotiates its supply contracts in the coming weeks with British Petroleum, Gulf and Shell. "One has to be realistic in understanding that security of supply and availability has a price," Kuwait Oil Minister Ali Khalifah Sabah said in a recent interview with the authoritative Middle East Economic Survey, "and if we are given that price, we will provide supply."

Some OPEC oil producers also are seeking to require oil companies to process their crude at certain refineries, to buy certain amounts of less desirable crude, and to buy processed oil products from the producing nations' own refineries. Kuwait's new oil corporation, for example, plans to double its own refining capacity in the next three to four years.

All this has reduced the role of the major oil companies, which once controlled the oil from the well to the gas pump. It has also increased both the profits and political power of OPEC producers, individually if not necessarily as a group.

Calderon of Venezuela said the long-range strategy also includes plans to increase OPEC aid to developing countries and reopen the OPEC dialogue with its major industrial nation customers. Ministers of Saudi Arabia, Venezuela, Algeria, Kuwait and Iraq and the governor of the Central Bank of Iran agreed here on strategy. It will be submitted to a special meeting of all 13 OPEC oil ministers this spring and to the OPEC heads of government summit marking the group's 20th anniversary this autumn in Baghdad.

The aid to developing countries would include grants and loans to ease the impact of higher oil prices, according to Calderon and the dialogue with industrial nations would include requests for increased transfers of Western technology to OPEC and other developing countries in return for OPEC assurances on oil supplies and prices.

In the absence of a unified price policy, the adoption of a proposal for gradual increased was seen by one analyst here as largely "window dressing," that overlooks the fact that in the present confused world market, "OPEC is as much uncertain of what it should as the rest of us."