In announcing that the US would quickly begin using crude oil from its 400 -million-barrel strategic petroleum reserve in the event of an oil cutoff from the Persian Gulf, the Reagan administration is sending a clear signal to Iran. Iran, the administration is saying, does not have as much leverage over Western oil supplies as it is implying by its threats to close off the Strait of Hormuz.
The administration decision is sound. Indeed, the underlying purpose of the oil reserve has always been that it should serve as an ''insurance policy'' to help prevent the United States - and indirectly its European allies and Japan - from ever again being held hostage to the dictates of Middle East oil-producing nations. That happened during the embargo by the Organization of Petroleum Exporting Countries back in 1973.
Use of the oil from the reserve in the event of a Gulf cutoff could buy the US up to 45 days of breathing space. It would also ward off the fear-induced hoarding that occurred in the 1979 oil crisis. The new policy, a clear departure from the administration's previous position that the strategic reserve would be tapped only as a last resort in any world crisis, underscores once again how vital it is that the US move ahead as quickly as possible to fill the reserve. There have been complaints that the administration has moved too slowly in seeking to fill the reserve.
All the current talk about using the reserve also underscores the mounting concern with which the administration is viewing the 42-month-old Iran-Iraq war. The war, no matter how it turns out, continues to have the potential to cause serious problems for the West. If Iran wins, the US would face the possibility of a fundamentalist Islamic empire hostile to Western interests stretching from the Syrian and Jordanian borders across to the Soviet Union. If Iran loses, on the other hand, there is the danger of that nation's slipping into anarchy, and the Soviets coming in to pick up the spoils.
As noted by Mideast expert Shireen T. Hunter on today's Opinion page, the stakes in the eventual outcome of the war are enormous. Could either side actually do what it has been unable to do during the past 42 months, namely, achieve a breakthrough? Both sides appear eager to try. Iran has launched a new human wave offensive, including sending young children into combat, some of them believed to be only 9 or 10 years of age. Iraq appears to be trying for a breakthrough by using poison gas, according to the US State Department, as well as by threatening to interdict ships using Kharg Island, Iran's major oil export facility.
Iran has vowed that if its shipping is interdicted it will close off the Strait of Hormuz, through which 20 percent of the West's oil exports come.
The United States must steer a very careful course during this period in its relations with both Iran and Iraq. It obviously has ample reason for wanting Iraq to hold its current defensive lines against the new Iranian offensive. If Iraq can hold its positions, that might eventually lead to calls for some form of settlement within Iran itself, as the casualty figures mount. At the same time, the US should be clear that it views any use of poison gas with the deepest revulsion.
The West has been trying to arrange a negotiated settlement for some 40 months now, but always without success. If the Iraqis can hold their lines, perhaps the Iranians will yet find some formula for a de facto cease-fire. But the ''ifs'' in Mideast scenarios are open-ended. Any number of unexpected situations could yet occur in that conflict. So in that sense, the Reagan administration has taken a prudent step in looking to the strategic oil reserve to offset Gulf oil shipments.
In dealing with the up-to-now stalemated Iranian-Iraqi war, having time on your side means everything.