Mexico, the leading non-OPEC oil supplier to the West, is cutting back on exports of crude oil in an attempt to halt the slide of petroleum prices, Energy Ministry officials confirmed here today.

Mexico's oil export slowdown, its first, is scheduled to begin Thursday. It will reduce its foreign sales by "less than 10 percent," Energy Minister Francisco Labastida Ochoa reported upon his arrival here yesterday from Europe. Labastida attended last week's OPEC-sponsored preliminary meeting of oil producers in Geneva and Mexican observers are attending the current emergency OPEC session there as part of consultations on ways of stabilizing prices.

Labastida announced that Mexico's export reduction will remain in effect "during the time that is necessary," the Energy Ministry's chief spokesman, Oscar Ramirez Suarez, said today in a telephone interview. Press reports here today that the production cut would continue for three months "are merely speculative," Ramirez said.

However, Mexican officials are said by industry analysts to hope that OPEC's pledged production drop of 1.5 million barrels per day and traditionally high winter oil demand will combine to halt price erosion soon.

Autumn storms in the Gulf of Mexico often interrupt oil loading, reducing Mexico's exports in the year's final months. Last December, Mexico's exports fell below 1.3 million barrels daily, in contrast to the 1.56 million barrels per day averaged during the first six months of 1984.

Skeptics here have suggested that Mexico's announced export cut could translate into a drop not significantly more than the inevitable decline caused by gulf gales.

But if Mexico were to inform customers that it was mandating real contractual reductions, analysts say, the move would represent its strongest gesture yet of support for OPEC, which lauded Mexico at its last full ministerial conference in Vienna on July 10 for having "pursued a policy of partial self-restraint in production with a view to supporting OPEC's price structure."

Dependent on crude sales for 70 percent of its export income, Mexico for two years has kept exports close to its self-imposed ceiling of 1.5 million barrels daily, nearly half of which is exported to the United States. Mexico emerged in 1982 as the main oil supplier of the United States. It accounted for 14 percent of U.S. petroleum imports last year.

Salvador del Rio, public relations director of Pemex, the state oil monopoly, said in a telephone interview today that he could not confirm reports that Mexico has informed clients that their expected November crude allotments have been trimmed.

A U.S. Embassy official here said American diplomats have not been told of Mexican plans to reduce oil exports to their U.S. customers. "We are not aware of anyone being informed by Pemex about reductions," the official said.

If Mexico were to slice exports by 10 percent it would cost the country more than $120 million monthly -- hard currency needed for interest payments on the country's $96 billion foreign debt.

Banamex, the state-owned commercial bank, recently projected Mexico's 1984 oil export earnings at $15.2 billion, while debt servicing alone is expected to claim $11 billion of the country's foreign exchange earnings this year.