Pennzoil Co. is investing $15 million to prove that fast-moving auto mechanics, using its oil, can help Pennzoil sell an estimated 10 million gallons a year.

Pennzoil, based in Houston, is finding its new market in the national chain of oil-change and lubrication centers operated by Jiffy Lube International of Baltimore.

Jiffy Lube, a fast-growing franchise operation, has 320 centers across the country. Under the new $110 million financing package completed this week, Pennzoil joined Jiffy Lube in a program to build another 325 Jiffy Lube centers.

The deal includes a guarantee that Pennzoil oil will be used at all Jiffy Lube centers.

W. E. Welcher, vice president for sales development at Pennzoil, noted that 645 Jiffy Lube centers, each servicing 50 cars a day, could use about 10 million gallons a year.

But he said he could not estimate the dollar value to his company.

The Jiffy Lube financing had two steps.

First, a $100 million, three-year real-estate partnership was formed among Pennzoil, which will own 50 percent, and Jiffy Lube, which will take a 10 percent share.

Additional partners will take the remaining 40 percent. Of the $100 million, $30 million will be in equity, $70 million in long-term debt.

Of the $30 million, Pennzoil will put in $15 million, Jiffy Lube will contribute $3 million and other parties will add $12 million, said John Sasser, chief financial officer at Jiffy Lube.

The money will be used to build the 325 new centers. The partnership will lease the buildings to Jiffy Lube, which will sublease them to the operators of the Jiffy Lube franchises.

An estimated 20 Jiffy Lube centers operate in the Washington area.

The real-estate partnership was formed, said Sasser, because Jiffy Lube had made it known that it needed real-estate financing. "We needed it to grow the company," he said. "That was the primary way to go."

The other part of the financing is a $10 million deal in which Shearson Lehman Brothers placed 12 percent, seven-year notes with an investor group led by Bridge Capital Investors, which includes Allied Capital, Connecticut Mutual Life and other partners not yet named.

Jiffy Lube had 120 franchises in March 1984 and 208 in March 1985. W. J. Hindman, chairman of Jiffy Lube, said the company's goal is to have 1,200 centers operating by 1990.