If you fancy conspiracy theories and believe that the big oil companies move in lockstep against the consumer, consider the industry's policies on credit cards:

* Exxon Corp. and Amoco Oil Co. take credit cards or cash, but charge customers 4 cents a gallon more for credit-card purchases.

* Shell Oil Co. takes both, but charges the same price to both credit-card and cash customers.

* Gulf Oil Co. used to charge a fee for credit-card purchases, but now has the same price for both credit-card and cash sales of gasoline.

* Texaco Inc. charges its dealers a fee for credit-card sales, but doesn't encourage them to pass it on to customers or to offer a discount for cash purchases.

* Atlantic Richfield Co. has done away with credit cards altogether and has slashed its prices as a result.

So much for unanimity. And every company is convinced that it's doing the right thing and, in some cases, has the numbers to prove it--even if they contradict those of a competitor.

Arco, for instance, has seen its share of the nation's gasoline market soar from 3 1/2 percent at the end of 1981 to 5 1/4 percent, moving it into a strong fourth place among the nation's gasoline retailers. But Shell boasts that its policy of accepting any and all credit cards has gained it 2 1/2 million new credit-card customers.

"If you ask Arco, there's no question that you ought to go to cash only. If you ask Shell, there's no question that you ought to offer credit-card service at the same price," says John Adkins, retail service and advertising manager at Exxon USA, the marketing division of Exxon Corp. "If you ask me, you've got to give the customer a choice."

And Adkins, echoing his competitors' attitudes about their differing strategies, adds: "No question we've done the right thing."

About the only thing that the combatants in the great credit card vs. cash war can agree on is that it's far from decided. And it's providing a glimpse of the new competitiveness in gasoline marketing.

"School's still out," Ron Hall, senior vice president for marketing at Gulf Oil Corp., says of the controversy.

The debate turns on feelings toward credit-card customers. Those companies charging the same rate regardless of method of payment say they don't want to antagonize their credit-card customers by charging them more, even though it costs the companies to process credit-card transactions.

"Our feeling is that the credit-card customer tends to resent paying more for his gasoline than a cash customer," says Richard R. Dickinson, senior vice president of marketing and transportation at Texaco Inc.'s gasoline marketing division, Texaco USA.

But officials of Exxon and Amoco say that by offering a discount to cash customers, they've gained an edge in the price wars that have raged at the gas pumps.

Nobody else in the business, however, has taken as radical a step as Arco, nobody else has had as striking results, and nobody else has anything too nice to say about Arco.

It's been about a year since Arco announced plans to phase out credit cards and go solely to cash sales, and the results in increased market share seem to bear out the strategy.

But competitors gripe that Arco has slashed its prices far more than the savings from the dropping of credit cards--about 3 cents a gallon, by Arco's reckoning--would seem to justify. Competitors say that Arco is undercutting their prices by as much as 12 cents a gallon, and they're not sure how Arco is doing it.

"Arco has been super-competitive on pricing. They basically have bought large volume with very low pricing in the past year," says Dickinson. "The credit card was really a minor part."

"There's no way I can understand how they can reduce the price that much," says William J. Bittles Jr., vice president of retail sales at Shell Oil Co. "If we reduced the price 9 or 10 or 12 cents like Arco did, we'd be out of business tomorrow."

Arco, for its part, is cagey about how it's managed to undercut competitors by so much. Edward G. Reilly, the company's vice president of marketing, says that, in addition to the 3-cent-a-gallon saving from termination of credit cards, the company also has passed on to dealers other price cuts he won't explain.

But his competitors speculate that the company is wringing every possible price advantage out of its large supply of Alaskan crude oil, its comparatively limited marketing area (it's estimated that Arco does half its business in two states, Pennsylvania and California, leading to significant savings in transportation and marketing costs), and just plain aggressive marketing.

Arco can't stay away from plastic forever, though. As technology advances gasoline marketing, electronic pumps that read credit or debit cards to dispense gas to customers and then bill them probably will lead the company back into some sort of sales to card-holding customers, Reilly concedes.

In the meantime, Big Oil is fragmented on the credit-card vs. cash question, increasing the choices for the motorist looking for a fill-up. "It's an accounting nightmare for the dealer," says Hall, "and I think it's confusing for the motorist."