Motorists across the country will feel the pinch of a growing gasoline shortage this month, according to Energy Department and oil industry executives.

There will be less gas available than there was last May, longer lines at service stations and more weekend closings.

A high-ranking Energy Department official warned yesterday that gasoline supplies during May could run 10 to 15 percent below demand.

Analysts are not optimistic about supplies for the summer peak-driving season just adhead. "I don't see any improvement over the next few months," said the Petroleum Industry Research Foundation's John Lichtblau.

Other Energy Department officials and some industry sources, however, remain skeptical about the extent of the shortage and the reasons for it. They point out that the government's data on supplies in the hands of the major oil companies and gasoline wholesalers are limited. They suspect that supplies are being withheld from the market now in anticipation of genuinely tighter market conditions and still higher prices later on.

In any case, DOE's Economic Regulatory Adminstration, which oversees gasoline allocations, says that gasoline dealers this month will receive an average of about 12 percent less gasoline than they received a year ago under the Department's complex regulations.

Meanwhile, according to an authoritative oil industry newsletter, oil companies have cut dealer allocations for May an average of 17.2 percent below what they got last year.

Actual deliveries run in excess of the official allocation figures because of special exemptions the government grants farmers, the Defense Department, emergency services and others, such as public transportation agencies.

Only two major oil companies, Getty and Marathon Oil, will give their dealers 100 percent of their scheduled allocations for May. Shell, the nation's largest gasoline marketer, will provide its dealers in the East 80 percent of their allocations, and those in the West 85 percent. Other major oil companies such as Exxon and Chevron will offer dealers 80 percent of their allocations, compared with 95 last month.

Mark Emond, editor of the Lundberg Letter, a gasoline industry trade publications, said yesterday in a telephone interview, "There are all kinds of ameiiorating factors, but the situation is still serious."

Jack Blum, a Washington attorney representing the Independent Gasoline Marketers Association, said that panic buying and "tank-topping" were major contributors to the gasoline lines that President Carter found in Los Angeles during his visit last weekend.

According to Standard Oil Co. of California executive Dale Brooks, in dustry-wide data indicate that in automobile-dependent California, the size of the average gasoline purchase has dropped from eight gallons to three gallons in the last month. The smaller - and thus more frequent - purchases mean that more gasoline than usual is being held in automobile gas tanks, resulting in a loss of 160,000 barrels a day from normal gasoline inventories.

Another indication of consumer fears over gas shortages in southern California is that supplies of lockable gasoline caps virtually sold oout last week.

Meanwhile, oil refinneries are operating at what some observers say is an execessivley low production level - 58.5 percent of capacity in recent weeks compared to 90 percent during the latter part of 1978.

Whatever the reasons for the shortage, one thing is certain: According to the Indepedent Petroleum Association of America's records, the wholesale price of gasoline rose 13.8 cents a gallon between January 1 and April 30. By way of comparison, during 1974, the year of the Arab oil embargo, wholesale gasoline prices rose 25.7 cents a gallon in the entire 12-month period.

Under the standby gasoline coupon rationing plan, now facing an uncertain future in Congress, the president would have the authority to call for gasoline rationing if there were a 15 to 20 percent shortage of gasoline based on government projections of demand.