MEXICO CITY, JULY 30 -- Mexico and Venezuela today pledged to maintain a discount oil sales program for Central America, but rebuffed Nicaraguan requests to receive crude on still more favorable terms.

Venezuelan President Jaime Lusinchi, concluding a four-day state visit here today, announced jointly with Mexican President Miguel de la Madrid that the oil supply program known as the San Jose Pact will be extended as of Aug. 3 for its eighth straight year.

Under the San Jose Pact's current terms, eligible countries are required to pay 80 percent of their oil bill in cash. The remaining 20 percent of their crude oil imports are financed with low-cost loans.

Through the San Jose Pact, Mexico and Venezuela supply up to 130,000 barrels of oil daily to several Caribbean islands and all the Central American countries except Nicaragua.

Nicaragua, contending that it cannot afford the pact's terms, has recently asked for Mexican and Venezuelan oil at a substantially higher discount. But Lusinchi, at a press conference today, said flatly that "there are no possibilities {of supplying oil} outside the framework of the San Jose agreement."

In recent weeks Nicaragua has stepped up efforts to secure new petroleum supplies to compensate for what its government said was a reduction in shipments from the Soviet Bloc, Nicaragua's main oil source for the past two years.

Venezuela and Mexico, the hemisphere's only major oil exporters, are also leading members of the Contadora Group, which is seeking a diplomatic solution to the Central American conflict. The oil program has enhanced the Contadora Group's negotiating leverage in Central America, diplomats said.

That oil-powered leverage no longer extends to Nicaragua, however. Venezuela cut off crude supplies to Nicaragua in 1982, complaining that the Sandinista government owed it more than $20 million. Mexico continued sending oil until 1985, suspending shipments when Nicaragua could no longer service an oil debt that, according to official sources, then surpassed $500 million.

At first the Soviet Union made up for the loss in oil to Nicaragua. But, according to some informed diplomats, while Moscow has guaranteed about 40 percent of Nicaragua's 1987 oil demand, it appears to have informed Managua that it will not cover Nicaragua's projected fuel shortfall. The diplomats described this as an effort to force the Sandinista government to find alternative sources.

"It seems clear that the Soviets will ultimately supply more oil if Nicaragua cannot get it elsewhere, but the Soviets would prefer to see Nicaragua also getting oil from places like Venezuela and Mexico," a European diplomat said today.

But even the concessionary terms of the Mexican-Venezuelan oil supply pact are too costly for "a country that is under attack from the U.S. government" and can therefore buy oil only "on very soft and favorable terms," Nicaraguan Energy Minister Emilio Rappaccioli said in Managua this week.

Nicaraguan President Daniel Ortega sent the same message in a personal letter Tuesday to Lusinchi and de la Madrid, saying the two leaders' Mexico City meeting would be "a propitious occasion" to consider "new forms of economic cooperation in our region, especially in the energy field."

For the past two months, diplomats from several countries affiliated with the Contadora Group have discussed a possible economic aid plan that would channel raw materials and foodstuffs from elsewhere in Latin America to Central America. As originally conceived by some participating diplomats, the aid plan's centerpiece was to have been a renewal of Mexican and Venezuelan oil shipments to Nicaragua.

The regional aid plan will be discussed further by Contadora diplomats in a meeting in Brazil on Aug. 10 and 11, Mexican Foreign Minister Bernardo Sepulveda said yesterday.

Henry Ruiz, the Cabinet minister who coordinates foreign aid, said in Managua Tuesday that Nicaragua has secured commitments for 3.4 million barrels of the 5.4 million barrels of oil that he said it needs this year.

The Soviet Union, Ruiz said, has promised to deliver 2.1 million barrels during 1987, the same amount he said it sent last year. But Ruiz said other former East Bloc suppliers have not responded to Nicaragua's 1987 requests.

The Warsaw Pact petroleum is of Soviet origin and was probably sent on to Nicaragua at Moscow's behest, knowledgeable diplomats said.