Standard Oil Co. (Indiana), the nation's sixth-largest oil company and the first of the industry's top 10 to report third-quarter earnings, said yesterday its profits rose 2.6 percent from those of a year earlier.

The company, commonly known by its Amoco brand name, said "significantly lower" profit margins on the sale of petroleum products overseas and "lower chemical profits due to the recession" held its third-quarter earnings in check.

The Chicago-based Amoco said it earned $448.9 million ($1.53 a share) in the third quarter, against $437.7 million ($1.50) in the third quarter of 1979. Sales rose to $6.5 billion from $5.1 billion.

In the first nine months, Amoco said higher gas and oil prices helped push earnings up about 33 percent to $1.51 billion ($5.16) from $1.14 billion ($3.89) a year before. Sales rose to $20.1 billion from $14.1 billion.

Amoco said it recorded a 21.7 percent return on shareholders' equity in the year ended Sept. 30. According to Business Week magazine, U.S. oil and coal companies recorded a 25.7 percent return on equity, while all industries tallied a 15.9 percent return, in the year ended June 30.

The Weyerhaeuser Co., meanwhile, cited damage from the May 18 eruption of Mount St. Helens in its report yesterday that third-quarter earnings were down 78 percent from the same period a year ago.

Even without the expenses related to the volcano's fury, third-quarter earnings fell to $68.8 million (52 cents a share), compared with $131.6 million ($1.03) for the same 1979 period, the giant forest product company said.

With the "extraordinary charge," however, earnings were $32.8 million (23 cents). Sales for the quarter fell to $1.11 billion from $1.17 billion last year.

The report listed sales of $3.4 billion in the nine months ended Sept. 28, a 4 percent increase over the $3.3 billion reported in the same period in 1979.

Even without the Mount St. Helens losses, however, earnings fell to $277.4 million ($2.14) in the first nine months of the year, compared with earnings of $402.7 million ($3.17) in the same 1979 period. The earnings decline was 42 percent to $241.4 million ($1.85) with the extraordinary charge taken into account.

Several transportation companies also reported their quarterly earnings yesterday.

Trans World Corp., owner of Trans World Airlines and the largest transportation company in the United States (based on sales), said yesterday its net income rose 189.1 percent over that of a year ago as profits from a bond exchange swelled its earnings.

Northwest Airlines Inc. said in St. Paul, Minn., meanwhile, that its earnings fell 7.8 percent in the quarter.

TWA said in New York that, including the effects of the debenture exchange, it earned $119.7 million in the quarter, up from $41.4 million a year ago. Per-share earnings rose to $6.99 from $2.28, and sales climbed to $1.4 billion from $1.3 billion.

Northwest said it earned $17.5 million in the third quarter, down from $19 million a year before. Per-share earnings fell to 81 cents from 88 cents, but sales rose to $481.7 million from $354.1 million.

In the first nine months, TWA earned $82.3 million on sales of $3.8 billion, against profits of $52.4 million on sales of $3.3 billion a year earlier. Per-share earnings rose to $4.08 from $2.57.

Northwest said it earned $1.4 million on sales of $1.2 billion in the first nine months, against profits of $70.5 million on sales of $977.1 million a year earlier. Per-share earnings fell to 6 cents from $3.26.

Burlington Northern Inc. yesterday reported third-quarter earnings of $40.0 million, up 140 percent from the $16.6 million reported in the same period in 1979.

Earnings per share in the third quarter ended Sept. 30 were $1.41, compared with 53 cents per share during the corresponding period in 1979. Consolidated revenues rose 19.7 percent to $1.04 billion from $867 million in the same period a year ago, BN reported.

During the third quarter, Burlington Northern's railroad generated pretax income of $32.4 million on revenues of $852.1 million, compared with a pretax loss of $22.2 million on revenues of $692.6 million during the 1979 third quarter.