AT ITS PEAK, in 1981, trade in oil was transferring income from the industrial countries to the Third World at a rate of about a quarter of a trillion dollars a year. Energy conservation and a softening price brought that flow down to well under $200 billion by last year. And the further drop in oil prices this month from $27 a barrel to somewhere around $20 will -- if it sticks -- knock off another $45 billion or so. At $20 a barrel the transfer of income from the industrial countries to the others -- or, more accurately, to a select few of them -- will be about half the level of 1981. Those numbers are quite large enough to make a difference to the whole world's economy.

When the price of oil was rising, it had the effect of a tax increase on the countries that imported it. Now that the price is falling, it will have the effect of a tax cut. It will push toward business expansion. That's good for the world in general, but particlarly for people who are unemployed. There are now 31 million of them in the industrial countries, most in Western Europe. Cautious European governments now have an opportunity to lean toward faster expansion without fear of inflation. That alone is sufficient reason to celebrate the $20 barrel of oil.

But this good news brings certain anxieties with it. Strains on the banking system are increasing, because banks have lent a great deal of money on oil production. Here in the United States, the bankers and the federal regulators ought to be able to contain the trouble. But the impact abroad will be harder to manage. For those countries that depend on their oil exports and are already deeply in debt, those debts will now be much harder to carry.

Among the OPEC countries and the other large exporters, it's always important to distinguish between those that have large populations and the desert states that do not. Saudi Arabia can live comfortably on its accumulated surpluses for quite a long time. Libya may have to scale down its plans to change the world, but not all Libyans -- let alone their neighbors -- would regret that. Among the importing countries with large populations, things are more serious.

For Mexico, Indonesia and especially Nigeria, a falling price of oil has a direct and immediate impact on the standard of living. Those countries are entitled to help, the most useful kinds of which will be delivered through those two big buildings on 19th Street, the World Bank and the International Monetary Fund. Will the Bank and the Fund have the resources to carry out their jobs? That depends on the rich countries, led by the United States.

With a lower price of oil, they can afford it. Americans and Europeans are entitled to rejoice at the prospect of more jobs and higher incomes at home -- but not at the cost of fewer jobs and deeper poverty in countries that lie closer to the Equator.