The Post overlooked an important factor in its coverage of potential petroleum shortages in Nicaragua {"Soviets Expected to Reduce Oil for Nicaragua," June 4}. Exxon, the nation's largest oil company, owns and operates Nicaragua's only oil refinery. Frequent breakdowns at this facility were recently cited by Sandinista commandant Henry Ruiz as a second cause of possible shortages.

The refinery is only kept operating with Exxon's help. Thus we witness the bizarre situation where crude is donated by the Soviet Union, refined by Exxon and placed in Soviet tanks, trucks and helicopters to fight U.S.-supplied forces in our own hemisphere. Because Exxon plays a role so critical to the Sandinista war effort, it is allowed to take out its profits in dollars, an opportunity denied to all other multinationals.

Economic sanctions against Nicaragua have widespread support, including the leadership of both parties in Congress. Exxon's continued operations in Nicaragua, which the company claims are legal, undercut this U.S. policy in dramatic fashion.

Exxon can no longer defend its relationship with the communists in Nicaragua. If it refuses to leave immediately, the administration should tighten the embargo to force Exxon out. PETER T. FLAHERTY Spokesman, Exxon Out of Nicaragua Coalition Washington