Several major oil companies, including Gulf Oil Corp., the nation's fifth largest, yesterday reported sharply higher first-quarter earnings.

Gulf's profit for the first three months of 1980 rose 56 percent to $389 million ($1.99 a share) from $249 million ($1.28) in the first quarter of last year. Revenues climbed to $7.8 billion from $5.6 billion.

The company said its foreign earnings fell 37 percent in the quarter as oil-exporting countries -- primarily Kuwait and Nigeria -- restricted production and sold less of their output through major companies such as Gulf. p

But profits in the United States soared 140 percent to $265 million because of higher margins on gasoline and other fuels, the company said.

Conoco Inc., the ninth largest U.S.-oil producer, said its earnings more than doubled to $328.6 million ($3.05 a share) from $161.8 million ($1.50) a year earlier as revenues increased from $2.8 billion to $4.3 billion.

Tenneco Inc., which is considered the tenth largest oil company but also is a conglomerate with major shipbuilding and other interests, reported a 45 percent increase in earnings from $123 million ($1.17 a share) last year to $178 million ($1.62) this year.

Revenues rose from $2.447 billion in 1979's first quarter to $3.316 billion.

Marathon Oil Co., 17th among U.S. oil firms, said its profits rose 33 percent from $104.5 million ($1.73 a share) to $139.1 million ($2.30).

Ashland Oil Co., which ranks just below marathon in sales, said its profits in the first three months of 1980 -- the company's second quarter of its fiscal year -- rose from $27.2 million to $47.4 million. Per-share earnings rose from 71 cents to $1.59 as revenue increased from $1.5 billion to $2 billion.

Ashland benefited in the quarter from sales of crude oil to it by several other large oil companies at the direction of the Department of Energy. Ashland was the U.S. company most affected last November when President Carter ordered an embargo on oil from Iran, and the sales were ordered to offset part of that loss of crude.

Earlier this week Exxon, the largest oil company, said its earnings for the quarter more than doubled to $1.925 billion, the greatest one-quarter profit any corporation ever has reported. Texaco's profits also doubled, and Standard Oil Co. (Indiana) said its earnings rose 65 percent.

Jerry McAfee, Gulf's chairman, defended his company's higher earnings, saying they are providing the cash flow and encouragement necessary to support the largest energy exploration and development effort in the company's history.

Gulf is embarked on a capital and exploration program expected to exceed $3 billion in 1980, or some $500 million more than was spent in 1979, McAfee said.

Despite its higher earnings, Gulf, like other international oil companies, has been squeezed by a change in the credit terms extended by most oil-exporting nations. Payment for oil generally now is due in 30 days instead of the 60 days that has been customary in the past.

Petroleum Intelligence Weekly estimates oil buyers now have nearly $22 billion tied up in financing their purchases. The highly respected international oil newsletter said that Gulf cited the OPEC credit squeeze as a primary factor in a $500 million cash shortage that led it to decide to sell 5 percent of its equity in Gulf Canada.

McAfee said Gulf's net income was reduced $31 million in the quarter by the new windfall tax on U.S. crude oil that became effective March 1. The chairman estimated the tax will cost Gulf about $500 million during 1980.

Conoco Chairman Ralph E. Bailey said the petroleum market conditions that contributed to his company's strong first-quarter performance aren't expected to continue for the remainder of 1980. He also cited the windfall crude tax and the oil import fee recently imposed by President Carter as factors that would reduce Conoco's profits later in the year.

The company's oil operations contributed most to the Tenneco earnings increase, but there also were major gains in its chemicals and shipbuilding businesses.

Meanwhile, Standard Oil Co. (Ohio), as expected following shareholder approval of an increase in the number of common stock shares, declared a two-for-one stock split. The company said the new shares will be distributed June 20 to holders of record May 16.