Energy Secretary James R. Schlesinger Jr. conceded yesterday that the Washington area is getting slightly less gasoline than other parts of the nation.

Schlesinger attributed this to his department's complex gas allocation system, a set of rules designed to equalize the burden of shortages while assuring that no gas station owner suffers unduly. But that system is not designed to prevent long gas lines, and may in fact have contributed to regional disparities in gasoline supplies.

Schlesinger's statement in a news conference contradicts assertions by officials in his Engery Department that the Washington area is receiving its fair share of the nation's scarce gas supplies. It tends to confirm suspicions of area members of Congress that this area is being slighted.

"The Washington area is receiving slightly less than the nation," Schlesinger told reporters. Despite this, he said, "Generally speaking, the capital area has received its fair share."

Schlesinger also discounted the significance of reports that the bulk storage tanks of several major oil companies here contain more gasoline than normal and that, while motorists wait in line, some pipeline shipments are being turned down because tanks are full.

He said that just because there is a large amount of gasoline in bulk tanks in Fairfax County does not necessarily mean that it is destined for consumption in the Washington area.

The Energy Department is investigating the reports, published in The Washington Post Wednesday.

Schlesinger said the area's proportionate share of gas supplies will vary month by month because of the way the federal gasoline allocation system works, but he did not explain precisely how this happens.

A spokesman for Exxon, the Washington area's largest gasoline retailer, with 450 of the 1,500 stations here, said the allocation rules forced the company to give Washington proportionately less in May and June than the rest of the country received.

At the same time, other areas of the country, like Florida, Georgia, the Southwest and other Sunbelt or highgrowth areas, received proportionately more gasoline from Exxon than it delivered nationally during those months, the spokesman said.

Here's how it works, according to Exxon:

The allocation rules basically require Exxon to distribute an equal percentage of its available gasoline supply in any given month to each of its service stations and other customers.

This month, for example, Exxon expects it can distribute 98 percent of the amount of gasoline it distributed during June a year ago. After priority customers, like farmers and the Defense Department, receive all they want, Exxon figures it has only 78 percent of last June's supplies left over to distribute to service stations.

So each Exxon station in the nation this month receives 78 percent of the amount of gasoline it pumped in June 1978.

There's a wrinkle:

To ensure that high-growth areas of the country aren't disciminated against, DOE in May changed the allocation rules slightly, so that each service station dealer must calculate the average amount of gasoline per month he sold during the period between October 1978 and February 1979.

He then compares that average figure with what he sold during June 1978, and if the average figure is 10 percent or more higher than the June 1978 figure, the dealer may choose the higher figure as his allocation "base."

In other words, the dealer will receive, this June, 78 percent of the higher average figure rather than 78 percent of the lower, June 1978, figure.

The Exxon spokesman said there are enormous distortions in this system. Florida, for example, has a tremendous tourist business from October through February. If a Florida dealer compares his October-February average with his sales in June 1978 - the hot season when tourists stay away - it is almost certain that the difference will be more than 10 percent.

This qualifies the Florida dealer for a higher proportion of the scarce gas supply than the average Washington area dealer can qualify for this June, even though the Florida dealer may not need all this gasoline this month.

Thus the allocation system is the cause of a central paradox of the current gasoline situation: Washington and other eastern cities have acute shortages, but other parts of the country - including rural areas less than 50 miles away - seem to have more than enough gas to meet demand.

Why can't some of the country gas be shipped to the city?

The answer, contained in several hundred pages of federal regulations, is that it is illegal for oil compaines to transfer gas from places with adequate soil companies to transfer gas from places with adequate sations center on an "equal application rule" intended to make each part of the country bear an equal share of the overall gas shortage. But, like all regulations, the gas rules have various exceptions, giving some users all they want regardless of shortages. The lose is the person without any exceptions - the average driver.

The regulatory scheme means that cities, which generally have more growth in demand and fewer drivers in the exception catergories than rural areas, are certain to suffer most from a national shortage.

The regulators do not consider this result a sign of failure. The Department of Energy, which created the allocation structure, in response to congressional directives, admits without hesitation that its allocation rules "do not prevent long lines, stockouts, and early closings at retail outlets."

"To understand the regs," said Frederick Khedouri, a congressional aide who has a reputation on Capitol Hill for understanding them better than most people, "you have to realize that they're not designed to protect the consuming public.

"The guiding principle is protection of the profits of gas station owners and wholesalers."

Viewed from that perspective, the allocation system seems to be working.

The rules refiners, who produce gasoline from crude oil, to give every wholesaler and every gas station the same share of total supply that it received during some earlier period. Right now the "base period" is the period one year ago.

This means that no gas station owner's market share will be less than last year's. It also means that stations with increased demand may not be able to get enough gas to meet it.

It is this requirement, that no individual wholesaler or gas station be short-changed, that prevents the refiners form shipping more gas to markets with the acute needs. Stepping up sales to the cities would require a reduction in sales to rural dealers, and that is against the rules.

The regulations provide, of course for special orders permitting a few changes from the year-earlier supply pattern.

But the Energy Department has been sparing in granting such orders.Observers in the industry and on Capitol Hill say this reluctance is a political phenomenon: The department does not want to be blamed for taking away from one governor's gas supply to alleviate a shortage in another governor's state.

Because the allocation system is based on year-earlier demand levels, it cannot reflect changes in relative demand over the previous 12 months. The recent amendments permitting higher allocations to rapidly growing regions were an effort to deal with this problem.

But these amendments have created new problems, including the development that the Department of Energy finally acknowledged yesterday: that Washington and some other cities may end up getting less than their fair share of the total gasoline supply. CAPTION: Picture, Schlesinger: It appears refining operations have been repressed." By Douglas Chevalier - The Washington Post; Chart, New Gasloine Rules