A former director of Mobil Oil Corp. testified yesterday that in 1979 he sought to have an outside investigation of the business dealings involving Mobil president William P. Tavoulareas and his son, Peter, but that no one else on the oil company's board supported his position.

George C. McGhee, a former U.S. ambassador to Turkey and West Germany who served on the Mobil board from 1968 until earlier this year, said he told the board that it was "intrinsically bad policy" for Mobil to have any dealings with a London-based shipping management firm in which Peter Tavoulareas was a partner.

McGhee testified during the eighth day of the Tavoulareases' $50 million libel suit against The Washington Post, which published two stories in late 1979 asserting that the elder Tavoulareas set up his son in the London company and that the firm had "since done millions of dollars in business operating Mobil-owned ships."

McGhee, a witness for The Post, said that three times during the 1970s he had opposed the arrangement under which the younger Tavoulareas' firm, Atlas Maritime Co., was managing the shipping operations of the Saudi Maritime Co. (Samarco), in which Mobil was a partner.

McGhee said that only once did another director also ask any questions about the arrangement when it was discussed annually at board meetings.

McGhee recalled that he told the board in 1979 that he opposed the arrangement "unless the element of nepotism can be eliminated." But he said, "I had no support" for his call for an outside investigation of the matter.

The elder Tavoulareas finished his testimony yesterday, conceding again, as he had on Wednesday, that on various occasions he discussed Atlas' business affairs even though he had pledged to Mobil officials that he would not make any decisions on Mobil's dealings with Atlas.

Under cross-examination by Irving Younger, one of the newspaper's attorneys, Tavoulareas acknowledged making several notations on a document that outlined the terms under which George Comnas, Atlas' senior partner during parts of 1974 and 1975, was ousted when Mobil officials became disenchanted with his work.

He also recalled that he got another Atlas partner, Ares D. Emmanuel, to agree that Atlas would reimburse Mobil for its costs in arranging a face-saving consulting contract for Comnas when he left the shipping firm.

Mobil officials have testified that Comnas performed poorly as a consultant and that eventually Atlas reimbursed $84,300 to the oil company, the money Mobil had paid Comnas.

Younger, in a far less contentious atmosphere than existed Wednesday, asked Tavoulareas a series of questions about Mobil's dealings with Atlas, but the Mobil president said that he generally did not know about the business arrangements until after they had been approved by other Mobil officials.