There was a pregnant moment during a meeting of President Reagan and his top advisers in the Oval Office the other day. The subject was the administration's apparently makeshift policy in the Persian Gulf, with particular emphasis on U.S. protection of tankers carrying oil to Japan and Europe.

Those at the meeting, besides the president, included Secretary of State George Shultz, Defense Secretary Caspar Weinberger and National Security Adviser Frank Carlucci.

At one point, a top official, noting that the 11 Kuwaiti tankers the president has promised to protect carry oil mainly to allied, non-American ports, suggested that the United States insist on military help from its allies. The least the administration should demand is financial assistance for what will otherwise be a ''free escort service,'' as the official put it.

Weinberger dismissed this suggestion with a bitter rhetorical question: ''Who's going to pay?''

No one could answer.

The hard facts of Japanese and Western oil consumption, which were discussed at the Oval Office meeting, cannot be ignored: Japan, France, Italy and West Germany depend heavily on Gulf oil but have showed little interest in helping protect the tankers that bring it out.

Great Britain, ironically, is the one steadfast ally in this crisis. The British, who in the past have supported even unpopular U.S. foreign policy initiatives, have no clear self-interest in Persian Gulf tanker protection. Quite the contrary: Britain is a net exporter of oil, and could conceivably reap a windfall in a world oil shortage caused by the closing of the Gulf.

But in secret bilateral meetings, the British have discussed possible ways they can help the United States in its self-appointed task of policing the Persian Gulf. The British may even add to the three warships they already have patrolling the risky waters.

Beyond global strategy and the principle of freedom of the seas, the United States has a minimal interest in securing the flow of oil through the Strait of Hormuz. Although 37 percent of our oil is now imported, only 4 percent of U.S. oil consumption depends on tankers sailing in the Persian Gulf.

Despite Reagan administration rhetoric to the contrary, the big danger in the past in the Gulf has come not from Iran or the Soviet Union, but from Iraq. Last year, for example, the Iraqis mounted three attacks on Gulf shipping for every two Iranian attacks. In 1985, the ratio was three Iraqi attacks to every Iranian one.

And while Iraq -- though apparently not intentionally -- was responsible for the only attack so far on an American warship, the Iranians have observed a strict policy of not attacking U.S. ships.

This can change drastically, of course, with the administration's planned escort service and the more belligerent talk on both sides. The most immediate danger is the Chinese-made Silkworm anti-ship missiles. U.S. intelligence believes Iran has purchased 40 from China, and will have the first one ready to fire by July 1. The missiles have a range of 57 miles -- nearly twice the distance across the Strait of Hormuz at its narrowest point.

There's another danger that hasn't been publicly discussed: Iranian suicide pilots, modeled after the Japanese kamikaze of World War II. Flying converted Swiss Pilatus PC7 crop dusters, they could fly in too low for U.S. ships' radar -- and, unlike the Silkworm missiles, their ''launching sites'' are not susceptible to a preemptive U.S. air strike. An Iranian pilot who defected described his fellow kamikazes as ''100 percent fanatics'' who would welcome the chance to immolate themselves in the explosion of a U.S. warship.

So there is an answer, after all, to Weinberger's acerbic question, ''Who's going to pay?'' The answer is: Americans.