Oil companies, fighting for position and profit in the increasingly competitive gasoline market, are trying to woo customers by offering feature-packed credit cards that let users do more than buy gas.

Mobil Corp. yesterday introduced a new "premium" credit card that includes the ability to get cash at Mobil stations, a travel reservation service and access to a "no-fee" Visa card from Chase Manhattan Bank, among other features. T.C. DeLoach, vice president and general manager of Mobil's Fairfax-based U.S. marketing division, said the new Mobil-Plus card "combines into a single card the services most preferred by our customers."

Other major oil companies have introduced similar cards in the past couple of years. Some of the companies also are experimenting with "debit" cards, which automatically take money from a customer's bank account to pay for a gasoline purchase. Mobil's new card includes this feature, and such developments could mean major changes in the way motorists buy gas in coming years.

But the new kinds of credit cards come with a catch: Unlike traditional oil credit cards, which have no annual fee, the new cards can cost anywhere from Mobil's $12 a year to more than $50 annually -- an effort by the companies to recover some of the costs of administering their credit card programs. For the same reason, many companies charge a few cents a gallon extra for gasoline bought with a credit card.

"Essentially, the credit card operations of oil companies have always lost money," said Spencer Nilson, editor of the Nilson Report, a newsletter that follows the credit card industry. "They would like to get all of their free customers onto a fee-paying basis."

The companies also hope the new cards will build brand loyalty -- and thus market share -- by encouraging holders to buy most of their gas from the company that issued them the card. "The total demand is essentially unchanged, so the broad long-term goal of every marketer is to get a bigger chunk of the same-sized pie," said Carol Edwards, a Mobil spokeswoman. "We're out there courting the customer more than ever before.

"We have to get the customer in there that many more times," she said. "The customer who might be coming in here two out of four or three out of four times -- this is an effort to get them to come to Mobil three out of four or four out of four times."

Mobil and the other oil companies are not dropping their existing free credit cards, but are hoping that the features included in the premium cards will encourage customers to trade up. "I think all the major oil companies are trying to worm around so they can charge a fee," Nilson said.

"We perceived a demand . . . that consumers wanted services that require a fee, and will pay a fee," said Phil Blackburn, a spokesman for Texaco Inc. The company's "Star Member" card, introduced late last year with a $20 annual fee, has attracted somewhere between 5 and 10 percent of Texaco's existing 7.5 million cardholders, Blackburn said. But he added that the figures are preliminary, and a better test will come when the cards come up for renewal at the end of their first year of use.

A spokesman for Shell Oil Co., which introduced its "Signature" card in 1985, said that card accounts for about 600,000 of its total 9.1 million credit card accounts.

Mobil officials yesterday argued that the benefits included with their new cards "far outweigh the fee," as DeLoach put it in a statement. The company throws in a Mobil travel guide, which retails for $8.95, and makes discounts available on lodging, air travel and car rentals, in addition to the other services offered.

The ability to get a Chase Manhattan Visa card through the Mobil-Plus card also appears to be something of a bargain. Chase Manhattan officials said the Visa cards would have no annual fee for at least the first two years, although the annual interest rate of 17.9 percent is higher than those on some other Visa cards.

Other oil companies also offer connections to Visa, Mastercard and other credit cards through their premium cards. Amoco Corp., the oil industry's largest issuer of credit cards, with 10 million cardholders, has perhaps the most extensive ties to other card programs: In addition to Visa and Mastercard, it also offers benefits from Diners Club or Carte Blanche.

In addition to its other features, the new Mobil card will offer the fullest test yet of the debit system for buying gasoline. Mobil already has been a pioneer in this area, allowing customers in the Washington area and elsewhere to buy gasoline with their automatic bank teller cards, with the purchases immediately deducted from the customers' accounts.

With the Mobil-Plus card, customers will be able to make debit-card purchases regardless of whether they have a bank card. Cardholders will be required to submit their checking account numbers when applying for the card, and transactions will be debited against those accounts through a service run by Chase Manhattan. Debit purchases of gasoline generally are charged at the same rate as cash purchases, or 4 cents a gallon cheaper than Mobil's credit card price.

Other companies have experimented with or implemented debit card programs, but they are not universal. Amoco, for instance, still is sporadically testing the idea, and a spokesman for Texaco said, "We don't think right now it's a necessary measure to take."