MEXICO CITY--It has been eight months since national oil firm Petróleos Mexicanos applied for a permit to buy U.S. light oil in a swap for Mexico's heavier blend, but the company says it is confident it will get U.S. Commerce Department approval and isn't looking to alternative sellers, a senior Pemex official said.

"The signs are positive," said José Manuel Carrera, head of Pemex's commercial arm PMI. "We continue to be respectful, but always very insistent that this happens as soon as possible," he said in an interview last week.

The U.S. maintains a ban on crude-oil exports dating back to the 1970s, but swaps are allowed when exports are balanced by additional imports. And while both the U.S. and Canada allow swaps, the subject is politically sensitive given how dependent the U.S. has been on massive oil imports in the recent past.

There is little precedent to measure the expected length of the approval process. The U.S. Commerce Department has declined to comment on the status of Mexico's request.

"We have given the required information, and we're waiting for them to finish their internal procedures, but we don't see any political or economic issues, just procedural ones," Mr. Carrera said.

Pemex wants as much as 100,000 barrels a day of the light, sweet oil from U.S. production in shale-rock formations in order to make its decades-old refineries more efficient. By mixing light oil with Mexico's heavier oil, the company's refineries will produce more diesel and gasoline per barrel and less lower-value products such as fuel oil.

Mexico's refineries were built when Pemex was producing lighter oil. But the mix has since shifted to heavier crudes with greater sulfur content. In contrast, many U.S. refineries were built to process heavier oils and already receive hundreds of thousands of barrels a day from Mexico.

Mr. Carrera doesn't doubt the U.S. will eventually give a green light to the swap. "The truth is that the treatment we have received and the signals we have received from the Commerce Department have always indicated to us that this is going very well."

Pemex sends around 700,000 barrels a day of crude oil to the U.S. out of its more than one-million barrels a day of overall exports. Mr. Carrera said the company has documented that the deal is in fact a swap of light crude for heavier crude, and not a net import of U.S. oil.

Pemex isn't looking to buy light, sweet oil from other exporters around the world because it is committed to further integration of the North American energy market among Mexico, the U.S. and Canada, Mr. Carrera said.

Examples of that integration include Mexico buying large quantities of natural gas, gasoline and other refined products from the U.S., while the U.S. and Canada also exchange energy products.

Likewise, Mexico last year opened its energy market to private and foreign companies for the first time in decades. Pemex has already announced its first private-equity deal and expects many more in the future. It is also holding its first auction of oil and gas blocks this year thanks to the energy overhaul.

"What we are constructing here are great avenues of energy links between Mexico and the United States so that movement is more fluid," said Mr. Carrera. "This petition for a license to import light U.S. crude is very consistent with the rest of the program we are working on with the U.S."

Mexico's swap request is separate from efforts by some U.S. lawmakers to lift the oil-export ban outright. Mr. Carrera said Pemex has no opinion on that since it is an internal matter for the U.S.

Write to Laurence Iliff at laurence.iliff@wsj.com

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