Treasury Secretary Donald Regan takes a more optimistic view of the world economic outlook than do officials of the International Monetary Fund, and will outline that theme at a meeting of the IMF and World Bank in Libreville, Gabon at the end of the month.

At a briefing yesterday on the Gabon conference, Treasury Assistant Secretary for International Affairs Marc Leland said that Regan "is not as grim" as IMF Managing Director Jacques de Larosiere, "because of the general outlook for the United States. Once our [economic recovery] program gets through, it should lift everybody."

Leland also said that Regan would leave tonight for Saudi Arabia to attend the regular annual meeting of a U.S.-Saudi joint economic development commission, with a refueling stop in Bonn, which will allow for a chat with West German Finance Minister Hans Matthoeffer. After Saudi Arabia, Regan will make stops in Kuwait and the United Arab Emirates on the way to Gabon.

One possible source of irritation during talks with the Arab states is the delicate issue of admission of the Palestine Liberation Organization to the IMF-Bank meetings.This question is bound to come up again in Gabon, but Leland said yesterday that "our position is clear: We will oppose the PLO, as participants or observers, as vehemently as possible, even more so than in the past."

When pressed to say how the opposition would be "even more" than shown by the Carter Administration, Leland said only that he meant the opposition would be "as least as much" as in the past.

The basis for a Gabon debate on the world economic outlook was touched off by de Larosiere last week in a San Francisco speech. He predicted a long stretch of high inflation and slow growth for the industrial nations, and a difficult period for the Third World nations in financing international payments deficits aggregating as much as $100 billion next year.

Leland said that the IMF Interim Committee and the joint IMF-World Bank Development Committee would have the general economic outlook on their agendas for Gabon, but he did not anticipate any specific actions to flow from the meeting.

He acknowledged that one important issue to be discussed is a proposal by some nations for an additional allocation of Special Drawing Rights (SDRs), the paper international asset created by the IMF for its member nations. But he indicated a negative U.S. attitude, saying the question is whether present world liquidity requires a new allotment of SDRs. A formal U.S. position has not been taken, he added.

The Treasury official also refused to say whether the abstension by the American Executive Director of the IMF last week, when the agency voted a loan to Granada, indicated that the U.S. would oppose loans to countries the U.S. finds on the wrong ideological wave-length.