Northern Virginia legislators, clearly divided on how to pay the region's share of Metro costs, today postponed a decision on whether to seek an area sales tax in addition to the gasoline sales tax already proposed by Gov. John N. Dalton.

While the 27-member delegation generally praised the Republican governor's proposal to split a 4 percent tax on wholesale gasoline prices between Metro and highway construction needs, area legislators had a number of questions about how much that tax would raise and whether it could be applied to the transit system's operating costs.

And uppermost in their minds during an afternoon meeting here was concern about whether any tax increase would ultimately be approved by the General Assembly.

"I'm not sure the governor's proposal will fly," cautioned Sen. Charles L. Waddell (D-Loudoun), who said he had been getting negative reactions to it from his Assembly colleagues.

Sen. Adelard L. Brault (D-Fairfax), the delegation's chairman, had gone into the meeting hoping to get a majority of the legislators to endorse a one-cent regional sales tax increase. He urged that Dalton's tax proposal won't be enough to pay Metro's growing operating deficit, expected to climb to $38 million by fiscal 1981.

But Brault dropped any attempt to forge a delegation consensus on the sales tax, expected to raise about $42 million a year, after hearing skeptical comments.

"If gas prices continue to escalate, they're going to have more money than they know what to do with," said Sen. Richard L. Saslaw (D-Fairfax), who suggested that some of that gasoline tax windfall could be applied toward the Metro operating deficit.

The delegation also wanted to know whether Dalton's gasoline tax proposal would satisfy federal requirements that Northern Virginia find a "stable and reliable" revenue source for its Metro operations. That question remains unresolved.

"I can't give you a straight answer," said Richard S. Page, Metro's general manager, who came here today to talk to legislative leaders and the delegation. He said that answer must come from the U.S. secretary of Transportation, and suggested the delegation contact that federal official.

Page warned the delegation that Metro's financial troubles would only worsen if the Assembly failed to approve either the Dalton proposal or other state aid measures.

"You can add at least 15 percent (to the cost) for every year completion of the system is delayed," he said.

Del. David G. Speck (R-Alexandria) urged the delegation to focus on passage of the gasoline tax and to worry about interpreting federal requirements later.

"If it walks like a stable and reliable source, and talks like a stable and reliable source, then let's call it a stable and reliable source and let the secretary of Transportation say it's not," Speck said.