Foreign bankers and diplomats said today that Mexico's decision to seek only $6 billion from overseas lenders this year is realistic, but the country must enact economic reforms before receiving new loans.

Mexico had planned net new borrowing of $2.5 billion from private banks and $1.6 billion from government and multilateral lenders in 1986. As a result of the plunge in oil prices, which will cut its export earnings, Mexico announced yesterday that it will ask its creditors for provide an additional $2 billion.

The announcement came 10 days after President Miguel de la Madrid declared in a nationally televised speech that the oil-price drop would cost Mexico $6 billion in anticipated export revenue.

This shortfall will be offset in part by $800 million in savings from lower interest rates, a $1.5 billion reduction in spending for imports, a $500 million increase above projections in nonpetroleum exports, and the cancellation of plans to boost foreign reserves by another $1.2 billion this year, the ministry said. Mexico now has about $5 billion in net foreign reserves, enough to pay for imports for four months, Western diplomats estimated.

Many smaller U.S. banks already are angry that their previous loans to Mexico have been rolled into long-term rescheduling packages and will resist pressure to contribute more, financial obervers here said. "We are counting on the U.S. government to help us raise the funds we need," a New York banker said.

In his Feb. 21 address, de la Madrid said foreign banks will have to make "sacrifices at least equal to those already made by the Mexican people." Mexico must halt "the vicious cycle of indebtedness" and reduce its servicing payments "in accord with its ability to pay," the president warned.

Some foreign bankers, interpreting the speech as a demand for radical and innovative debt-payment formulas, said they expected a request for postponement and eventual conversion into principal of part of the approximately $9 billion in interest payments Mexico owes this year.

Finance Minister Jesus Silva-Herzog and Public Credit Director Angel Gurria will discuss the borrowing request with U.S. and international finance officials in Washington Wednesday and Thursday and with bankers in New York Friday, officials said.

"The proposals are moderate and measured," a U.S. diplomat said, adding that U.S. officials "expect to work closely with Mexico in coming weeks" in arranging the details of the new loan package.

Following negotiations for new money, Mexico is expected to press for better repayment terms for old loans. In August, most of Mexico's public-sector debts were renegotiated at the lowest interest rate charged any major Third World debtor, and a further reduction "would inevitably be a trend-setter for other debtors," a British debt analyst here said.

Before it gets its $6 billion, however, Mexico must convince creditors that it is implementing substantive economic reforms, bankers and diplomats here stressed. Officials contend they already have dramatically reordered the economy, but they are being asked to sell more subsidized state businesses and amend laws keeping foreign-owned businesses out of such potentially attractive investment areas as electronics and petrochemicals.

Mexico also first must conclude a new loan pact with the International Monetary Fund before it is granted new credits or payment deferrals, they said. Mexico's talks with the IMF were broken off following the recent collapse of world oil prices.

Mexico's budget deficit, considered by U.S. government analysts and many private Mexican economists to be a more serious problem than its foreign debt, will remain as large as it was last year, the Finance Ministry tacitly acknowledged Monday.

Last year's deficit was caused in part by the government's moves to stimulate the economy during midterm congressional elections and two difficult statehouse races. This year, the ruling Institutional Revolutionary Party is facing gubernatorial challenges in 13 of Mexico's 31 states as it begins to gear up for the 1988 presidential contest.