In months, hundreds of new families will converge on the Washington area. They won't be here rallying or demonstrating -- they'll be living here as part of Mobil Oil Co.'s relocation of its U.S. marketing and refining division from New York City to Fairfax County.

Mobil Oil officials said last week that by the end of next month they expect to begin moving employes into the two wings of the eight-story office building on the 130-acre tract bounded by the Capital Beltway, Arlington Boulevard and Gallows Road. The whole transfer will take about five to six weeks, a Mobil official said.

Mobil will be the fourth corporation employing more than 1,000 persons to enter the county. The others are American Telephone & Telegraph Long Lines division, the metric division of the Mitre Corp. and surburban offices of the Washington-based Planning Research Corp., according to the county's economic development office.

Just to handle the Mobile move, consisting of about one-third of the company's New York staff, Mobil has set up relocation offics in New York and Fairfax and has hired Area Consultants Inc. to help the workers find new homes and schools and cope with the move.

"The greatest problems have been perceptual," said Robert Stahl, founder of Area Consultants. One perception is that Virginia as part of the South has inferior schools and that housing in the Washington area is extremely expensive, Stahl said. "Frankly, it scared them."

So far, Fairfax County has been the "overwhelming choice" as a home by Mobil's employes partly because it will be close to work and "it has a lot to offer," Stahl continued.

The transfer of such a large number of workers won't place a burden on Fairfax County services, said Buck Buchanan, one of the county's economic development agents. For example, a $12 million project to build an overpass to carry Gallows Raod traffic over Arlington Boulevard near the Beltway is supposed to begin next year to ease traffic jams brought about by the Mobil employes. The project will be funded by federal, state and county governments.

"Taxes will more than offset the cost" of community services necessary for the additional people moving into the area, Buchanan said. County officials estimate that Mobil's move will generate $616,000 annually in real estate taxes and $1.55 million in other taxes. Cost of services such as fire and police protection, and water and sewer services will cost the couty $400,000 a year, Buchanan said. The county also expects little impact on schools.

A 1976 report on the Mobil move estimated that the county will accrue $8 million in revenues over a 10-year period compared with a cumulative deficit of $3.5 million if the Mobil site had been used for single family homes.

Fairfax had planned on the company moving all of its headquarters operations from New York. Those offices employ 3,500 persons.

But two years ago Mobil decided to keep most of its headquarters staff in New York. The company was under "very heavy pressure" not to move out of Manhattan, a company official said at the time.

Mobil reportedly was a prime target of a New York City task force, headed by former Deputy Mayor Osborn Elliott, that was lobbying businesses to stay in the city.

The Fairfax office will consist of two eight-story buildings connected by a cylindrical core housing elevators and building services. An adjacent three-story building will have a cafeteria, medical services and a training center.

Because some Mobil employes decided not to move with their division, numerous clerical jobs are available at the Fairfax office. Mobil already has established an employment office in Virginia and has placed help wanted ads in local newspapers, a Mobil representative said. However, she wouldn't release the number of jobs available because the company feared that would produce a rush of job applications the office couldn't handle, she said.

So far, more than half of the workers relocating from New York have commitments on either renting or buying apartments and homes, the Mobil representative said.