The United States should limit its involvement with the corrupt and ineffective regime of Zairian President Mobutu Sese Seko, according to a Senate Foreign Relations Committee staff report that prompted last week's committee vote to reduce U.S. aid to Zaire.

The gloomy conclusions by three committee staff members, who reported to the senators March 24 after their trip to Zaire last August, outweighed low-key, pro-Mobutu lobbying by the Israeli Embassy and domestic backers of Israel.

Congressional sources said the lobbyists had asked the senators to show their gratitude to Mobutu for his recent move to reestablish diplomatic relations with the Jewish state.

In explaining its vote last Thursday to halve the proposed $20 million in U.S. military assistance, the Senate committee told the Senate that it is seeking to send "a clear message" that reforms in use of U.S. military aid to Zaire are imperative.

Without going into detail, the panel cited reports that some equipment provided to Zaire has been diverted to private use.

The staff report, a portion of which was read at the markup session by Sen. Paul E. Tsongas (D-Mass.), said Mobutu has converted one of his U.S.-supplied C130 military transports for use as his personal plane with a special "executive capsule" installed at his own expense.

"Mobutu employs the aircraft frequently and has even learned how to fly it," the report said.

At the time of the Senate staff visit to the Zairian military airfield near Kinshasa, a second C130 transport was being loaded with cattle to be flown to Mobutu's private ranch in order to improve the quality of his herd.

The other five of the seven C130s bought with U.S. military credits in 1977 were all out of action, according to the staff study. One had been destroyed in a crash, two were grounded due to crash damage and lack of spare parts and the others were being serviced or prepared for service in Italy, which won a general servicing contract for the Lockheed planes.

Citing several other examples of corruption and ineffectiveness, the members reported that "it is widely accepted that he Mobutu has managed to amass a legendary personal fortune at the nation's expense."

At the same time, staff members Alison Rosenberg, David L. Johnson and Philip Christenson noted that real wages for the average Zairian citizen have declined "steadily and drastically" despite the country's mineral riches.

Because of corruption higher up, even military units are not regularly paid but are often forced to live off the land at the expense of the local population, according to the report.

This has created growing discontent and a situation in which "it is increasingly risky for the United States to expect perpetuation of the status quo," the staff report said.

The risk of sudden change in an unknown direction in the large, strategically located country is compounded, the staffers said, by the public identification of the United States with the regime, an identification fostered by Mobutu.

To bring about reforms through outside pressure, "the first step" is forging an American consensus about the limits of U.S. involvement with Mobutu, according to the staffers. In the security assistance field, especially, "it may be that the only effective impetus for serious reform in Zaire will be a credible indication of U.S. willingness to disengage entirely," their report said.

On May 13, two weeks before the Senate committee deliberations, Mobutu responded to criticism and aid cuts by the House Foreign Affairs Committee with the announcement by his official news agency, AZAP, that he had decided to renounce U.S. aid. In view of the importance he attached to his U.S. connection, puzzled senior U.S. officials expressed doubt that Mobutu really meant it.

The following day, Mobutu announced that he was reestablishing diplomatic relations with Israel, making Zaire the first to do so of the two dozen black African countries that severed their Israeli relations at the onset of the 1973 Middle East war.

There was speculation among U.S. officials that, beyond its international implications, Mobutu expected his decision to enlist Israel on his side in Congress.

The Israeli Embassy and the American Israel Public Affairs Commission (AIPAC), lobbying arm of U.S. Jews, confirmed reports that their representatives had been active in Congress on Zaire's behalf following Mobutu's announcement about Israel.

But both groups described their efforts as low-key gestures rather than a real campaign. An AIPAC official said its activity had been undertaken at the suggestion of the Reagan administration.

With both the Senate and House committees having approved restrictive legislation, the State Department refuses to say whether Mobutu intends to reject American aid. The standard answer to all questions is that the matter is being discussed with Zaire "in diplomatic channels."