* USX Corp. reported a $394-million fourth-quarter...

* USX Corp. reported a $394-million fourth-quarter loss Tuesday, partly due to the falling price of oil and restructuring costs in its steel division.

USX earned $267 million in the last three months of 1990. Revenues fell 22% to $4.87 billion in the latest fourth quarter from $6.22 billion.

The company does not report earnings per share as a whole because its stock is split into separate classes for energy and steel.

The company’s Marathon Oil group lost $229 million, or 90 cents a share, in the latest quarter, contrasted with earnings of $216 million, or 84 cents a share, in the year-ago period. Sales fell 22% to $3.6 billion from $4.6 billion.

The fourth-quarter loss was due largely to a $260-million accounting charge against income to lower the value of inventory in light of falling oil prices. In the year-ago period, the value of inventory was increased $140 million.

Marathon’s results were affected by lower worldwide oil prices and reduced refined product margins, said USX Chairman Charles Corry.

The U.S. Steel Group, which also includes operations such as mining and real estate, lost $165 million in the fourth quarter, or $3.25 a share, compared to earnings of $51 million, or 99 cents a share, in the year-ago period.

Sales fell 19% to $1.3 billion from $1.6 billion.

The 1991 results were affected by restructuring charges of $80 million, including a $40 million pretax charge for the proposed shutdown of the South Works in Chicago, and various litigation and environmental charges of $49 million.