President Carter's new budget-balancing plan, approved late Thursday by the House Budget Committee, ran into new difficulties yesterday as the chairmen of the House and Senate tax-writing committees indicated they have reservations about his tax proposals.

Senate Finance Committee Chairman Russell B. Long (D-La.) told reporters he is vigorously opposed to the $4.62 a-barrel oil import fee Carter has imposed and will not support the 10-cent-a-gallon gasoline tax increase the president is seeking to replace it.

At the same time, both Long and House Ways and Means Committee Chairman Al Ullman (D-Ore.) expressed pessimism about prospects for two other measures -- a plan to withhold taxes on interest and dividends, and another for $3.5 billion in minor tax and fee increases.

Failure to approve both these proposals would wipe out the projected surplus that the Budget Committee has recommended and throw the fiscal 1981 budget back into deficit, not counting expected revenues from the oil import fee.

Meanwhile, the Budget Committee agreed yesterday to alter its accounting procedures for the fiscal 1981 budget resolution it approved Thursday and to list the surplus as $5.5 billion, rather than the $2 billion announced earlier. l

The change reflects a decision to count the $3.5 billion in proposed new tax measures as part of the overall budget resolution, rather than earmarking the new money for possible future tax cuts.

The resolution still would set aside $10.3 billion in expected revenues from the oil import fee for possible tax reductions if the budget ultimately is balanced, in line with a last-minute amendment proposed by Republicans.

The new figures for the fiscal 1981 spending plan show $16.5 billion in spending cuts, with total outlays of $611.8 billion. With the revenues from the oil import fee included, the surplus would be $15.8 billion.

In a related development, Postmaster General William Bolger served notice that cutbacks the Budget Committee proposed in Saturday mail service would affect only delivery, and would not mean closing individual post offices. w

"I'm not even giving a second thought to closing post offices on Saturday, or stations or branches," Bolger said. "We can't eliminate our seven-day operation. We might be able to adjust here and there, but not eliminate it."

The Budget panel voted Thursday to slash funds for Saturday mail service and the current junk-mail subsidy, a move that was solidly supported by the Carter administration.

Meanwhile, administration sources said Carter has formally approved most of the specific cutback proposals the White House will send Congress, but is still not ready to make them public.

There has been speculation that the administration is withholding details of its plan to avoid angering voters in Tuesday's New York primary. Carter asked Congress a week ago to cut spending by $13 billion to help fight inflation.

Meanwhile, several congressional Republicans said the GOP support that enabled Budget Committee Democrats to push through their spending plan late Thursday may disappear when the resolution reaches the House next week.

Rep. Newt Gingrich (R-Ga.), a leader of the House GOP freshmen, said Republicans were dissatisfied with the tax-increase proposals in the budget and predicted that fewer than 15 members of his party would support it on the floor. Other conservative Republicans voiced similar complaints.

The Budget Committee vote approving the fiscal 1981 budget resolution included support, for the first time, from five of the panel's Republicans. But six liberal Democrats defected and opposed the plan.

The revenue-raising measures contained in the budget resolution call for eventual enactment of $3.5 billion in a variety of tax changes, ranging from higher cigarette and liquor taxes to a possible surcharge on high-income persons.

However, while the $3.5 billion target for new revenue would be binding if Congress approved the plan, the details would be left up to the discretion of the Ways and Means Committee, which then would have to propose new tax legislation.

Ullman said yesterday his panel "is certainly going to try to abide" by the budget resolution mandate, but he added that the Budget Committee's list of suggested tax hikes was "not realistic" and predicted that passage would be difficult.

Ullman also expressed misgivings about the Budget panel's call for raising $3.3 billion by withholding taxes on interest and dividends. "That's just going to arouse a massive resistance," he said.

The Ways and Means chairman also was cautious in discussing Carter's request for a 10-cent-a-gallon hike in gasoline taxes, hinting that no action was likely until sometime in 1981.