IRANIAN OIL, it now appears, was pouring into this country in great volumes during the early summer. That sharpens the dilemma for this country in dealing with Iran. There's an obvious case for Americans to refuse to buy Iran's oil. It's offensive to carry on business as usual with that supplier. The money goes into weapons and the prolongation of a murderous war that the United States is working anxiously to turn off. American ships are in waters threatened by Iranian mines. But to cut off American imports would require sanctions. American sanctions would inconvenience Iran, but as long as the world's thirst for oil remains at its present level, there would be other buyers. It would not be a crippling blow unless the rest of the world joined in -- and doing without a couple of million barrels a day of oil would mean the kind of shortages and price increases for which the world demonstrably has a low tolerance.

In July, it appears, this country imported more than 600,000 barrels a day from Iran -- more than from any other country but Nigeria. You need to know that July was not, in the oil trade, a normal month. The fighting in the Persian Gulf had set off a sudden surge of buying on the part of oil companies hastily building stocks for the coming winter. Now the atmosphere is less tense, and imports not only have dropped sharply but seem to have returned to their normal pattern. But the normal pattern means about 250,000 barrels a day from Iran -- less than half the July surge and only a third as much as this country generally buys from Venezuela or Canada but still a substantial amount of oil, representing something over $1.5 billion a year for the Iranian war effort.

It's possible to cut off that flow, but not easy. The United States' boycott of Libyan oil demonstrated the difficulties. For some time importers evaded the ban by taking Libyan crude to Europe for refining and then bringing the products into this country. To trace oil back through the international refineries is not simple.

In the Iranian case there would be another layer of complication. When Iran returned the American hostages in 1981, it made a deal with the United States. A tribunal was to be set up at The Hague to adjudicate American claims against revolutionary Iran and, as part of the same deal, this country lifted its sanctions against Iran. If the United States were now to re-impose them the Iranians could claim violation of that agreement and pull out of the Hague process, in which Americans have much at stake.

For the present, oil sanctions are more useful as a threat that can be invoked in the future if Iran continues, recklessly and obdurately, to pursue the war. They constitute a heavy weapon, cumbersome and expensive, to be invoked only when all attempts at negotiation and persuasion have finally failed and left no choice but the policies to which the United States has been pushed in dealing with Libya