AS THE MASSIVE costs of imported oil throw countries into rising trade deficits, the international system of money is coming under great strain. The International Monetary Fund has calculated that the industrial countries together had a trade deficit in 1980 that was half as high as in 1979. The IMF's job is to lend to both rich and poor countries that are having trouble with their foreign debts. Those loans are designed to buy time for adjustment, avoiding crises and depressions in weakened economies. But this winter the IMF has been running rapidly toward the limits of its lending ability.

Now Saudi Arabia has announced a massive loan to the IMF -- some $10 billion over the next two years. It's a notably helpful contribution to the world's economic stability. The mirror image of the oil-imported countries' debts are, of course, the oil-exporting countries' surpluses. The ideal solution would be, clearly, heavy lending by the surplus countries directly back to their customers -- particularly their customers in the Third World. But the exporters object that they are not equipped to carry on much of that kind of commercial lending. They have generally left their money in American and European banks. That leaves the banks carrying all of the risks, and the poorest and most heavily indebted countries are having increasing trouble finding commericial credit. But if direct lending by the surplus countries is not likely, the next-best answer is the kind of lending to the IMF on which the Saudis have now embarked.

In return, the Saudis will gain greater power in the IMF's policy decisions. It will now rank sixth in votes, after the United States, Britain, Germany, France and Japan. There is some apprehension in this country that Saudi Arabia will use its new influence in causes like the campaign to give the PLO observer status in the IMF's meetings. But the Saudis have generally been cautious in their use of their great financial power, and there's no reason to expect any great change now.

This period of very large trade deficits for most countries, and very large surpluses for a few, is temporary. In time, the world will work its way to a new balance. But that process is not necessarily benign. Part of the adjustment process is the very slow growth and high unemployment now visible in most of the industrial world, including this country. In the poorer countries, the transition threatens much greater hardship. It can be limited by the ability of the IMF to keep lending to countries as they adjust, and that ability has now been enhanced by the Saudi loan.