Amoco and Mobil, which supply almost 300 of the Washington area's 1,500 service states, announced yesterday that they will slightly increase gasoline deliveries to stations here and nationwide in July.

But the modestly encouraging news came amid indications that gasoline supplies may be tighter here this weekend than ever before. Oil industry representatives also said that gasoline supplies will continue to be tight throughout the summer.

"We don't see any easing of the supply situation," Joe T. McMillan, Exxon's manager of domestic supplies, said in a press conference yesterday. Exxon, which has 450 stations here, has not announced its supply plans for July.

"The situation should remain as it is through the end of 1979," McMillan said. He said that "extraordinary conservation measures" will be "the solution for the rest of the year."

The Washington area supply picture for July is not yet clear. Sunoco, with 110 stations here, announced that it will slightly decrease gasoline deliveries to stations in July.

Shell and Texaco, with a combined total of 425 stations here, have not announced their supply plans for July.

"I'm telling you, we've very happy it [the allocation for July] is increasing, but until the gas lines disappear we're not all the happy," said Buz Warfield, Amoco's Washington area manager.

Gasoline lines in the Washington area continued to show improvement yesterday compared with the situation a week ago, when a system of odd-even sales went into effect in an effort to cut down lines that stretched for blocks and tied up motorists for hours.

Nonetheless, a large number of stations said they had run out of gasoline or were about to in the next day or two, increasing the likelihood that this weekend's supply situation will be severe.

"We haven't had any gas since last Wednesday and we won't get any until the second of July," said Jasper Parrish, a mechanic at Wilson's Exxon, 3801 Georgia Ave. NW.

"I'd say the problem is worse than in any month since the energy crisis began," said Vic Rasheed, executive director of the Greater Washington-Maryland Service State Association.

"The dealers are running out quicker this month . . . Basically, they just overpumped."

This weekend, Rasheed said, "is going to be a rough one right through until after the Fourth of July holiday in the middle of next week."

The modest July increases by Amoco and Mobil were made possible by increased purhcases by the two oil companies of crude oil and refined gasoline on the international spot markets, company spokesmen said.

In a related development, the oil industry mounted increasing attacks yesterday on the complex U.S. regulations that determine allocations of scarce gasoline supplies. Politicians here contend the system has caused the Washington area to suffer more during the shortage than many other areas.

"Events of the past few weeks illustrate that the shortage is being borne disproportionally by the nation's urban areas," Mobil's vice president for marketing and refining, Allen E. Murray, wrote Energy Secretary James R. Schlesinger yesterday.

Murray demanded that the energy department eliminate a rule that allows Southern and Western areas to receive more fuel than the Northeast of the mid-Atlantic states.

Murray also demanded an end to the vast and expanding system of "priority users," such as farmers, some truckers and others, being able to get all the gasoline they want. He said priority users should be limited to what they received a year ago.

Despite the planned July increases, Amoco and Mobil both with be supplying less fuel nationwide than they were during July a year ago - Amoco about 90 percent of the previous year's figure and Mobil about 97 percent.

This June, Amoco supplied only about 80 percent nationwide of what it supplied during June a year ago, and Mobil supplied about 95 percent of the year-previous figure.

As the situation worsened on the Atlantic Coast yesterday, Maryland Gov. Harry Hughes moved to make 200,000 gallons of state emergency setaside gasoline available to 50 stations in the Baltimore and Washington areas. Pennsylvania Gov. Dick Thornburgh ordered odd-even sales into effect in his state today. A similar odd-even plan is scheduled to begin in Delaware on Friday.

The fuel crunch was spreading well beyond metropolitan areas. Police in Cumberland reported gasoline lines and traffic tieups yesterday for the first time in that western Maryland town.

Closer in, a substantial but unknown number of stations in the Washington area already have run out of their June allocation of gasoline, according to both the American Automobile Association and the local gasoline dealers associator here. Those stations will not be pumping again until at least July 2, when the first of the July deliveries arrive.

There was also continued talk by some individual station owners of shutting down later this week - not because they are out of gasoline but as a protest against what they said are shrinking profit margins in their business. The shutdown proposal did not appear to have the backing of local dealer associations, and it was not clear how extensive any attempted shutdown may be.

According to an AAA survey, 6 percent of Washington area service stations have run out of gasoline for June and up to 30 percent of stations in Maryland outside the Washington area have run out of one or more grades of fuel for the rest of the month.

Bill Lowey at Taylor's Sunoco, 6360 New Hampshire Ave. in Takoma Park, said a delivery scheduled Tuesday never arrived and he is out for the remainder of the month unless he gets an emergency allocation.

John Conner, owner of Connecticut Avenue Amoco at 5001 Connecticut Ave. NW, said he is down to his last 750 gallons for the month.

Why are the stations running out? Rasheed of the service station association said "some are trying to be fair with odd-even" by remaining open every weekday and pumping more gasoline than they should. Others, he said, have had their allocations unexpectedly reduced in mid-month by the oil companies, leaving them short at the end of the month.

Rasheed urged motorists to buy gasoline outside the metropolitan area where it is generally more plentiful.

Rasheed said his 1,300-member organization plans to file a request Friday with the U.S. Energy Department's hearings and appeals office to increase the Washington area's July allocation by 10 percent. The department has emergency power to transfer limited amounts of fuel to "adversely affected" areas from areas with surpluses.

"We certainly we adversely affected," Rasheed said. "In May, we got 89 percent of the allocation we got in May 1978, while Hawaii got 126 percent."

He said he thinks his association's planned request for the federal action has a greater chance of succeeding than a lawsuit filed Tuesday by the state of Maryland. In that suit, Maryland accused the Energy Department of giving the state less than its fair share of gasoline under a complex federal allocation formula.

In another action, Gov. Hughes ordered state employes who commute to work in state-owned cars to start paying $10 to $40 every two weeks for the privilege, effective Aug. 1. Paycheck deductions will vary according to the number of miles driven by the employes.

Also contributing to this article were staff writers Kenneth Bredemeier, Patricia Camp, Joseph Contreras, Dennis Kneale, T. R. Reid and Edward D. Sargent, and staff researcher Reginal Fraind .