President Carter intensified his administration's efforts to smooth over three weeks of sharp challenges by Israel to U.S. plans for the Middle East by hosting Israeli Ambassador Ephraim Evron yesterday at an unusual working luncheon in the White House.

Standing beside Evron in the White House driveway after the 90-minute session, Secretary of State Cyrus R. Vance insisted to reporters that "there has been no change in our policy toward Isreal."

Evidently seeking to counter staements by Israeli Foreign Minister Moshe Dayan on Tuesday that U.S. policy toward Israel was changing because of U.S. reliance on Arab oil producers, Vance said this country's support for Israel remains "unshankable."

Both Vance and Evron refused to offer any details of the meeting, which also was attended by national security adviser Zbigniew Brzezinski.

U.S. officials portrayed the meeting as having contributed to clearing the air. But Evron declined to say in response to a reporter's question that the meeting had succeeded in dispelling the Israeli doubts or resolving the main topics of recent conflict between Jerusalem and Washington.

The Israeli ambassador said instead that he would report to his government based on what the president had told him and also on what would follow.

Although expressed in a number of different ways, Israeli concern has centered particularly on what the Israelis see as an unclear American attitude toward possible contacts with the Palestine Liberation Organization.

PLO officials have sent a signal, repeated to the Carter administration, that they might be willing to accept publicly a new United Nations resolution that would repeat the terms of Security Council Resolution 242 and add a reference to Palestinian rights to self-determination, diplomatic souces report.

That signal was received in Washington about three weeks ago and touched off intense discussions within the administration over U.S. strategy toward dealing with the Palestinians, U.S. officials say.

Resolution 242 is a deliberately ambiguous document that formally halted the 1967 war and became the basis for peacemaking efforts since. It deals with Palestinians only as refugees, not as political participants in peacemaking.

Through a spokesman, Vance repeated for reporters yesterday an assurance he previously had given Sen. Richard Stone (D-Fla.) and the Israelis in private. The United States would veto a Kuwaiti-sponsored draft U.N. resolution that calls for a Palestinian state, the State Department said.

In response to a question, Stone said yesterday that he also had asked Vance if the United States would veto a resolution that did not go that far, but merely added a call for self-determination or something similar to the terms of Resolution 242.

"We are not negotiating such a resolution," Stone quoted Vance as having responded. The senator said that he did not press Vance for a less equivocal answer.

The Carter administration has said it will open contacts with the PLO if that organization accepts Resolution 242 and affirms Israel's right to exist. Doing so would not constitute a change in U.S. policy, but could create an explosive confrontation with Israel.

Not responding positively to a PLO acceptance of Resolution 242 would expose the administration to sharp retaliation by Saudi Arabia, which has put particular stress on the need for a U.S.-PLO dialogue and which last month agreed to Carter's plea for increased oil production to ease world markets.

The Saudis reportedly agreed to increase production about a million barrels of oil a day for three months. A review of that decision is expected in October - the month that Egyptian and U.S. officials repeatedly have siad is crucial.

October will be the sixth anniversary of President Anwar Sadat's Ramadan war against Israel, and U.S. officials have said it is vital to achieve the appearance of progress by then to convince the Palestinians to accept the Egyptian-U.S.-Israeli talks on autonomy for the West Bank and Gaza Strip territories.

Vance directed spokesman Tom Poston yesterday to deny Dayan's charge that "a real change" was occurring in U.S. policy because "the United States is concerned about problems of its economy, energy and the price of oil," and accordingly "is looking for an understanding with Saudi Arabia."

"The United States is doing nothing in the Middle East negotiations based on our need for oil," Reston said. "There is no linkage."

Evron's visit to the White House yesterday was arranged by Carter during a telephone call over the weekend to Israel's convalescing prime iminster, Meachem Begin.

An administration official said the meeting concerned both short-term and long-term issues, giving Evron a chance to hear U.S. policy defined personally by Carter. It was not, the official said, a negotiating session. CAPTION: Picture 1, Israeli Ambassador Ephraim Evron and Secretary of State Cyrus R. Vance meet with reporters after a White House luncheon at which President Carter sought to assure Israel of U.S. support. By Frank Johnston - The Washington Post; Picture 2, Carter and Evron enjoy a light moment during their discussion of U.S. policies. White House Photo