A federal court jury yesterday deliberated for another 7 1/2 hours over the $50 million libel suit brought by Mobil Oil Corp. president William P. Tavoulareas and his son, Peter, against The Washington Post, but went home a second night without reaching a verdict.

The jury, which now has spent 13 hours considering the case, gave no indication that it is deadlocked and sent no messages to U.S. District Judge Oliver Gasch requesting any clarifications about the massive evidence in the case.

Gasch called the jury, composed of three men and three women, back into his courtroom at 6 p.m. for a progress report. Jury foreman Geoffrey T. Mott, a librarian at the Library of Congress, told him, "Your honor, we have not reached a verdict yet."

"I'm always opposed to what some people refer to as a fatigued decision," Gasch responded and dismissed the jurors for the night. The judge told the jury to resume its deliberations at 9:30 a.m. today.

The complex case centers on two articles The Post published in late 1979 detailing the Tavoulareases' business dealings in connection with the creation of Atlas Maritime Co., a London-based shipping management firm in which Peter Tavoulareas is a partner. The Tavoulareases claim that the stories libeled them.

While the jurors pored over an estimated 160 exhibits in the case and considered the 16 days of testimony they heard, the principals in the case wandered the fourth floor hallway outside the courtroom in the U.S. Courthouse, read newspapers and books, made small talk and worked crossword puzzles to pass the time.

The elder Tavoulareas has been declared a public figure for purposes of the libel suit, but yesterday he had occasion to wonder how apt the description was.

A lawyer representing Gulf Oil was in the courthouse for what turned out to be an unsuccessful effort to win immediate approval of the oil firm's bid to merge with Cities Service Co. He met Tavoulareas, who promptly delivered a short discourse on oil company mergers.

The Gulf lawyer, after Tavoulareas introduced himself, innocently asked, "Oh, are you in the gasoline business?"

"I'm some public figure, huh?" Tavoulareas remarked to bystanders.