Amid howls of protest from Capitol Hill, the White House sought yesterday to jimmy a window for the Synthetic Fuels Corp. to approve more than $500 million in federal subsidies to two oil-shale projects before Congress abolished the agency.

Congress had forbade the action in a two-day stopgap spending bill signed by President Reagan Tuesday night. But the White House, seeking to appease two western-state senators with projects in the pipeline, contended yesterday that the spending measure expired at noon -- opening the way for the Synfuels board to approve the aid in an emergency session before Congress could pass another spending bill that would again close the door to further subsidies.

That startled Congress, which thought it had passed a bill that expired 12 hours later, at midnight. The dispute sent lawyers all over town scurrying for dictionaries to divine the meaning of "12 o'clock post meridian" -- or 12 p.m. Did that mean noon or midnight?

Enraged opponents of the Synfuels Corp. headed for the Senate and House floors in droves, accusing the administration of "absolute arrogance" and warning that Congress would not let the matter rest.

Sen. Howard M. Metzenbaum (D-Ohio) called the loophole maneuver "unbelievably callous and arrogant" and added: "How many other ways can we tell the administration to stop this wasteful spending of taxpayers' money? Not one penny more."

Rep. Silvio O. Conte (R-Mass.), who added the restrictive language to the spending bill, criticized the proposed subsidies and warned that the Synfuels board had "better be prepared to answer questions in court" if the aid was approved.

But the administration's attempted fiat already had fallen flat at the Synfuels Corp., where officials of the dying agency announced that there would be no emergency meeting to consider additional subsidies.

Agency officials said the administration's support for the subsidies was "weak." Congressional aides said the corporation's decision may also have been influenced by a letter from House Energy and Commerce Chairman John D. Dingell (D-Mich.) alluding to the "fiduciary responsibilities" of board members and the potential of criminal and civil liability for ill-spent federal money.

The episode was the latest -- and presumably the last -- in a frantic and unusually visible campaign to win more than $500 million in loan and price guarantees for Union Oil Co.'s project in Parachute Creek, Colo., and Geokinetic Inc.'s Seep Ridge plant near Vernal, Utah.

Sens. William L. Armstrong (R-Colo.) and Jake Garn (R-Utah), with the help of Senate Energy Committee Chairman James A. McClure (R-Idaho), fought hard for the subsidies, arguing that the administration had a long-standing commitment to the projects.

Late Tuesday, the senators won agreement from White House chief of staff Donald T. Regan, and Armstrong tried to push the projects through the Synfuels board that night before President Reagan signed the stopgap measure forbidding the action.

The Synfuels board balked, however, saying the agency needed formal authorization from the Office of Management and Budget. Yesterday, OMB director James C. Miller formally notified the House and Senate Appropriations chairmen that the administration "has no objection" to funding the projects.

"We need something a hell of a lot more formal than that," said Synfuels spokesman Graham Matthews.

Armstrong and Garn were seeking a $327 million loan guarantee for Union Oil, which considers the subsidy necessary to get the plant in operation so it can take advantage of $900 million in federal price supports already approved. The smaller Seep Ridge project was in line for $184.3 million in price and loan guarantees.

The dispute over the expiration time of the temporary spending bill became moot last night when Congress replaced it with a bill to provide longer-term funding. But according to the Government Printing Office style manual, 12 p.m. means midnight, 12 m. means noon and 12 a.m., presumably, does not exist.

"That's really weird," said a GPO spokesman.