Mobil Oil Corp. president William P. Tavoulareas, testifying yesterday in his $50 million libel suit against The Washington Post, tangled sharply with the newspaper's lawyer over the extent of his involvement in a shipping deal involving Mobil and a firm in which his son Peter had a financial stake.

The Mobil president acknowledged that on two occasions in 1974 he discussed details of a contract being negotiated between his son's firm and a shipping partnership partly owned by Mobil.

But he denied that in discussing the financial details of the contract he actually was helping to negotiate the terms under which his son's London-based firm, Atlas Maritime Co., would manage shipping operations for the Saudi Maritime Co. (Samarco), in which Mobil was a partner.

"I was free to negotiate," Tavoulareas said when asked about the terms of an August 1974 Mobil edict that he would not be involved in any decision-making affecting Atlas. "But I never remember negotiating."

At another point, the Mobil president said that he suggested various terms of the contract to Atlas and Samarco officials at a waterfront social gathering in Saudi Arabia in November 1974.

"I wouldn't call that negotiating," Tavoulareas said. "I call that like being a judge. I explained the proposal to one side and then the other." Tavoulareas said other details of the Atlas-Samarco contract were discussed at a luncheon in Geneva.

The extent to which Tavoulareas was involved in Atlas' affairs is crucial to the dispute over two 1979 Washington Post stories that Tavoulareas and his son allege were libelous. The newspaper's articles stated that Tavoulareas set up his son in Atlas in 1974 and that the firm had then done "millions of dollars" of business operating Mobil-owned ships.

Tavoulareas has denied that he urged George Comnas, the initial senior partner in Atlas, to include his then 25-year-old son Peter in the firm.

However, during a contentious, four-hour cross-examination yesterday by Post attorney Irving Younger, Tavoulareas acknowledged that Samarco had paid $4.1 million to Atlas from 1974 to 1979 for ship management services.

However, Tavoulareas said that the $4.1 million only represented gross revenues paid to Atlas, out of which the firm had to pay for its operating expenses.

He described the annual $600,000 management fee paid by Samarco to Atlas as "a very thin contract."

"The major part of the money Atlas has made has been from other contracts," Tavoulareas said. "It has not come from millions of dollars of Mobil money that I've somehow sneaked in."

As more than 100 spectators watched in U.S. District Judge Oliver Gasch's courtoom, Tavoulareas repeatedly questioned the premises of Younger's questions and sparred with the attorney over the meaning of seemingly ordinary words.

At one point, Younger asked the 62-year-old Tavoulareas, "Would you agree Atlas is the operating arm of Samarco?"

"I don't know what arm is," Tavoulareas retorted.

Moments later he conceded the point, telling Younger, "If you want to call it the operating arm, you can call it the operating arm."

Younger also drew an admission from Tavoulareas that five of his relatives work for Mobil, including two nephews, a niece, "a very distant cousin" and a brother-in-law. Younger did not ask him if he had any role in hiring them.

The six-member jury heard one other wit- ness yesterday, Mobil board chairman Rawleigh Warner Jr., who supported Tavoulareas' contention that he was not involved in Mobil decision-making affecting Atlas.