The 20 member-countries of the International Energy Agency today commissioned a special study of what effect the controversial U.S. decision to subsidize oil purchases on the European market may have had on prices on the volatile Rotterdam oil market.

This was the conclusion of a special meeting called to seek information from U.S. representatives about why the Carter administration had taken its action.

Agency sources said that several delegations expressed surprise that the Americans had taken their action without informing or consulting the other IEA countries, especially since it was announced very shortly after U.S. Energy Secretary James Schlesinger attended an agency ministerial meeting here last month.

There has been a major European outcry, led by France, against the U.S. decision, the only major western industrial oil-consuming nation that is not a member of the IEA. The French have been saying that the American action illustrates the uselessness of the agency and justifies their decision to stay out of it.

IEA Secretary General Ulf Lantzke accepted the assignment to present a report on the U.S. action's effect on Rotterdam and other "spot" markets for oil not traded under contracts to a meeting June 26 of the IEA's standing group on the oil market.

At today's session, he said it is already clear that it is hard to trace such effects. He recalled that oil on the Rotterdam spot market had jumped about $5 a barrel this week before the U.S. decision and that it jumped another $5 two days afterward. IEA officials quoted Lantzke as asking whether it could be determined that the second jump would not have taken place if the Americans had not made their move.

Dutch officials, who have legal authority over the Rotterdam market, the most important of its kind in the world, pledged that they would supply Lantzke with the maximum amount of information.

The U.S. delegation told the other members that the American subsidy program, technically known as entitlements, was only for 200,000 barrels a day for the four months ending Aug. 31, to make up for the abnormally low current U.S. stocks of home heating oil.

U.S. Ambassador Herbert Saltzman reportedly said in reply to questions by other delegation chiefs that he doubted that the United States would give up the program before Aug. 31 because it would take that long to reestablish the minimum heating oil stocks the Americans need to get through next winter.