There is a strong likelihood that Gulf Reston Inc. will sell its remaining interests in the 15-year-old "new town" of Reston in the coming months.

The oil company subsidiary, now headed by William Cox from the parent corporation, is reported to be poised to sell income-producing property it owns in the Fairfax County community for nearly $40 million. A reported sale in December to a group of West Coast pension funds did not materialize.

The likely purchaser now is a jointventure investment group headed by the Potomac-based Donatelli and Klein Inc. Louis Donatelli, president, said that Weaver Bros., a major real estate and mortgage banking firm, and the Mark Winkler Management Co., a realty firm in Northern Virginia, will participate in the transaction along with a "prime" (but unnamed) financial institution.

Properties involved include a highrise office building, a high-rise apartment building on Lake Anne, 10 lowrise office buildings, five garden apartment projects and four shoping centers.Donatelli said the negotiations are solid but may take several months to complete.

Two years ago, Donatelli and Klein Inc. and Mark Winkler Management Co. bought the 440-unit Northgate garden apartment complex in Reston.

The 7,400-acre, 30,000-resident new town, begun by developer Robert E. Simon in the early 1960s, was taken over by Gulf Reston Inc. in 1967. Last fall, Mobil Oil Corp. bought the remaining 3,700 undeveloped acres for $30 million.

Since then, Reston Land Corp., a subsidiary of Mobil Land Development Corp., has become Reston's developer -- but without taking any role in building or property management.

The new Reston Land Corp. (the Mobil subsidiary) has a development team headed by James Todd, Francis Steinbauer and James Cleveland, all of whom formerly worked for Gulf Reston. Steinbauer was in charge of the construction of most of the interior roads as well as the community's recreation and service facilities.

"We know who we are," president Todd said recently, "but the transition has been so smooth that many people are apparently unaware of the change.... We are in business to sell the land to complete the residential and commercial development, but we are not competing with builders in construction."

Despite the fact that residential Reston is only half built and commercial property only 40 percent developed, the public seems to think that the new town is completed, Todd said.

Todd, Steinbauer and Cleveland predicted that 1979 will be Reston's biggest development year. They point out houses are being built there by 30 builders, including Ryan Homes, the area's biggest homebuilder. Ryan will begin selling medium-priced town houses -- probably priced in the $60,000s -- in Reston later this year.

In addition, developers are planning to build a dozen industrial-commercial buildings in the community this year.

Three new commercial centers and six restaurants are also in the works and Sperry Systems plans to begin a 36-acre office-industrial complex that will eventually provide employment to about 1,000 persons. Reston firms currently have a total work force of 10,000.

With major interior roads completed and a new four-lane bridge carrying Wiehle Avenue over the Dulles airport highway, Restonians also are anticipating the mid-1980 completion on four parallel lanes of toll roads along the Dulles road. The toll lanes would connect to the new Rte. I-66 to give Reston commuters a direct route to downtown Washington.

Over the years Reston has acquired more than 2.3 million square feet of office and warehouse facilties. The U.S. Geological Survey is a major employer and there are 27 trade associations (including the Council for Exceptional Children and the American Newspaper Publishers Association) now located in the new town. Commercial development along both sides of the Dulles highway is expected to increase now that the parallel toll roads are virtually assured for use by 1984.

Todd and Cleveland said that interest in locating business and light industries in Reston has increased because of the improved interior transportation and the likelihood of better road connections to Washington and to Dulles, which is five miles west of the new town.

Reston has more than 11,000 completed houses and 1,400 under construction. Prices range from under $40,000 for some garden condominium apartments to more than $200,000 for houses located on one of the four lakes or near the two golf courses.

There are also 3,500 rental apartments. Todd and private broker Robert Howard (of Wellborn Co. of Reston) said that the resale market is strong and that there are few apartment vacancies.

Earlier this week the first elderly tenants moved into the new Fellowship House II, a subsidized high-rise in south Reston sponsored by a foundation of Lutheran laymen.

Reston's principal residential market is made up of resale housing which total about 1,100 a year, broker Howard said. They sell for an average of $75,000, he said.

However, there are few detached houses available for less than $100,000 and town houses make up the major share of the Reston housing resale market. Some of the larger town houses, both new and resale, are prced around $135,000 range.

While recent Reston growth has been mainly in the southern sector, a large chunk of land remains to be developed north of the first village, north and south of Rte. 606 and east and west of Rte. 602. Three more lakes, a sports center and a town center are in the planning stage.

Reston development is not expected to be fully completed for another dozen years. "But this should be the most exciting period," Todd said.

John Morris, a Reston attorney who recently completed a term as president of the citizens' association, agreed.

"They are following the land use plan," he said of the developers. "So far, the transition from one developing oil company to another has been quiet and undramatic. We have a good relationship between citizens and developer. I hope it continues that way."

If Reston, indeed, is completed by 1992, it will be about 10 years behind the finishing date targeted by the original developer, Robert Simon, who lost control of the town during a tight mortgage market period in the late 1960s. Gulf weathered the financial crunch of 1973-74 but then decided to get out of the land development business.

Now Mobil, which has five other land developments and also owns the Colonial Village apartment complex in Arlington, has its corporate hand on Reston's tiller. Todd is optimistic about the future: "Things never looked better," he said. "Only a major collapse of the national economy can stop us now."