At about 4:30 p.m. yesterday, Prince George's County lawyer Paul Nussbaum pulled his Lincoln Continental into the almost-deserted Transit Truck stop in Beltsville and filled up his tank without waiting behind a single car.

Nussbaum, who paid cash for a half tank of unleaded gas, didn't have to wait because he's a regular customer. Others are being turned away.

The operators of Transit Truck, who also own the Bi-rite gas station two miles down U.S. 1, say they are saving their gas these days for regular customers. It is a practice that, according to a Department of Energy spokesman, may violate federal rules.

Mary Ann Frymark, who with her husband, Herb, and their six sons runs the two stations mom-and-pop style, says she knows it might be against the law."But I can't see where they can make us sell to people who never bought from us before."

In past years, when gas was plentiful and nearby stations were underselling them, "we were almost put out of business," Mrs. Frymark says. The regular customers "are the ones who made the past sales. . . . We have to take care of them. If the government wants to come in here and say we can't, I'll fight it all the way."

The Frymarks' formula is this: any driver, of truck or car, can buy diesel fuel. Gasoline, however, is reserved for customers who have cash accounts, or, in the case of Nussbaum, have been patronizing the station for a long time. And, adds son Herb Jr., if friends come in, he'll sell them gas.

It wasn't always this way. Herb Frymark Jr. said that before gas supplies tightened this year, the stations would sell to motorists off the street.

Department of Energy spokeman Jack Vandenberg said yesterday that federal regulations apparently prohibit arrangements like the Frymarks'. The rules, adopted in 1973, say that stations must continue the business practices they used before May 15, 1973. "If you sold to the man on the street in 1973, then you have to continue to sell to the man on the street," he said.

The rules forbid refusing to sell "due to the absence of a prior selling relationship with the purchaser," he said.

Violations carry fines: a maximum of $2,5000 peer day per violation; if it is a willful violation, the maximum rises to $10,000.

Vandenberg said Doe's enforcement division is compiling figures on consumer complaints, and will make them public in a day or two.

The Frymarks say they've already heard from the federal enforcers. "We have a pile of complaints from DOE, on prices, commercial accounts, charge accounts," said Mrs. Frymark. She said the stations, whose supplier is Space Petroleum of Baltimore, have stayed within price increase guidelines.

Those prices have raised some motorists' eyebrows, however. The big Bi-rite sign advertises three prices: regular, 94.9, unleaded 95.9, and diesel, 52.0.

Inked in beside the diesel figure is the caveat: "1/2 gal." CAPTION: Picture, Beltsville truck stop notice means what it says, even if pump doesn't. By Gerald Martineau - The Washington Post