The Persian Gulf oil crisis will weaken the global economy over the next year, boosting unemployment and inflation rates slightly, but it is not likely to tip the world into a recession, the International Monetary Fund said yesterday.

This optimistic appraisal, issued prior to the annual meeting of the World Bank and IMF here next week, is based on the assumption that the price of oil -- at $33.18 yesterday -- will come down to $26 a barrel in the last quarter of this year, then decline to $21 by the fourth quarter of 1991.

By the end of next year, the IMF said, the price of oil is likely to be back where it was before the crisis and then continue to be level in real terms, that is, adjusted for inflation. The net damage, according to the IMF's annual World Economic Outlook, might be a half-point annual loss in the real growth rates of major industrial countries and a quarter- to half-point rise in the inflation indexes.

The agency said that Third World countries, taken as a whole, will actually benefit slightly from the oil price increase because there are several large oil-exporting countries among them. But a senior official warned that the IMF might seek to borrow from those Third World countries enjoying that profit and lend the money at below-market rates to others hard hit by the rise in prices.

Countries feeling the impact of higher oil should allow the price increase to be passed on to business and consumers and make no attempt to ease monetary policy, the agency said, to avoid inflation.

The report warned labor unions not to try to offset higher prices through "an escalation of wages and a subsequent wage-price spiral." Because an increase in oil prices involves a decline in national income, "real wages as well as profits will unavoidably be lower than they would have been otherwise," the report said.

Jacob Frenkel, director of research for the IMF, acknowledged that the continuing uncertainties relating to the Persian Gulf crisis could result in a much higher price of oil, upsetting the optimistic economic projections.

But if the IMF's expectations are realized, global economic growth would drop from 3 percent in 1989 to 2 percent this year, then recover to 2.5 percent in 1991. In the United States, real gross national product would fall from 2.5 percent last year to 1.3 percent this year and recover to 1.7 percent in 1991.

The prospective slight improvement in the U.S. economy next year was attributed to not only the assumed fall in oil prices in 1991, but also the recent decline of the dollar IMF officials said may trigger a resumption of larger exports next year.

Frenkel, responding to a number of questions implying skepticism, defended the projections as within reason and noted that even if oil prices were to settle at around current levels, the impact would be much less than during the oil "shocks" of the 1970s. Oil prices quadrupled in 1973-74, then tripled in 1979-80.

Ernesto Hernandez-Cata, a senior IMF adviser in charge of developing the oil projections, said he "did a lot of praying" while working them out. He acknowledged that actual oil prices "will depend on the military and political outcome" in the Persian Gulf, and added that the IMF did not pretend to have expertise in those areas.

Hernandez-Cata said, "The situation is at risk, but the {IMF} scenario is plausible" and consistent with a political settlement next spring. It is also consistent, he said, with a stalemate in the Persian Gulf that nevertheless allows Saudi Arabia and other countries to boost crude oil output by about 4 million barrels a day.