Sunmark Industries, the marketing division of the Sun Oil Co. of Pennsylvania, has sued the state of Maryland in federal court for passing a law banning the conversion of service stations to "gas-only" operations.

The suit contends that Maryland is violating the Constitution and that the law - passed in July - "is in conflict with federal antitrust policies."

Prior to passage, Sun had converted 11 of its 150 Maryland stations to "gas only" status - thations that provide only gasoline at a convenient location. Sun had planned at least 11 more conversions in the state.

Sun began converting the stations when in discovered that "revoluntionary changes in the gasoline marketplace dictated by consumer preferences have occured in the past several years," according to Sunmark president Harlan T. Snider.

He cited trends in auto manufacturing that led to "less frequent and more complicated service requirements for new cars. The trend is towards specialized repair facilities and do-it-yourself products."

Snider said many full-service gas stations are "now under-utilized and are being subsidized by consumers who come to service stations for a fill-up, not a tune-up."

The suit contends that the law was pushed through the Maryland legislature by the greater Washington/Maryland Service Station Association to protect the market position of a group of local dealers and will result in less efficient service to consumers.

"The pricing patters show that gasoline sold by 'gas-only' outlets is more economical than that sold through full service accounts," Snider said. "The Maryland consumer is being denied this saving by this law."