Relying on what may have been true yesterday, but doubtful today, the 21-nation International Energy Agency has rolled out yet another highly touted "World Energy Outlook," warning of the possibility of a new and devastating oil crisis in the mid- to late 1980s.

The underlying trend, the IEA said, points to a disappearance of the current worldwide oil glut by mid-decade. Demand would then surge ahead of available supplies by as much as "9 million to 21 million barrels a day by the end of the century."

All this got wide and respectful attention, including front-page play in The Washington Post.

Now, it's only prudent to cover all bases: The Western world certainly should have a contingency plan to deal with the possibility of another oil embargo or shattering event in the Middle East. We need, for example, to make sure that we keep an adequate Strategic Petroleum Reserve.

But the best answer to the IEA's worst-case scare story comes -- in an extraordinary way -- from a four-page advertisement in The Wall Street Journal for Friday, Oct. 29, extolling Saudi Arabia's rags-to-riches progress since 1933.

The ad contains this stark evaluation of the declining demand for oil and the prospect for still falling prices:

"During the last 18 months, it has become clear that the mighty oil weapon is rapidly vanishing. Falling demand among the oil-importing nations is not as temporary as it seemed . . .

"Standard Oil Company of California (SOCAL) forecast recently that there will be no real rises in oil prices before the late 1980s, and that demand for OPEC oil in 1990 is likely to be only half the estimates made four years ago. World recession, conservation measures and a search for alternative energy sources has backfired on the oil exporters."

Ads bought by national economic development agencies are usually self-serving puffery. This one in The Wall Street Journal is no exception, but it also contains a certain amount of candor. A WSJ spokesman said that what it calls "advertorials" were written by London free-lancers commissioned by The Journal to achieve "first-class newspaper quality." The copy was not cleared with the Saudis, The Journal told me.

In reality, much contained in the advertisement is not up to Wall Street Journal editorial standards. For example, The Journal's own excellent energy reporters would not likely have written that the Saudis "set out to create the current oil glut to hold the price down," because they know the Saudis in fact pumped as much oil as they could at peak prices simply to bring in the money.

But even a Saudi house ad is forced to admit -- which the IEA does not -- that the oil supply-demand situation has now changed dramatically. Says a critic of OPEC: "Some of it sounds like the stuff I've been writing myself."

By any calm appraisal, the oil glut is not a temporary flash in the pan, or "deceptive," as the IEA argues, but reflective of a totally new dynamic in the world oil supply-demand picture. That's what analysts such as historian John B. Kelly, Fred Singer of the Heritage Foundation and Eliyahu Kanovsky of Queens College, and Washington energy experts Philip Verleger, Lawrence Goldmuntz and Joseph Lerner -- all from differing perspectives -- have been saying for the last couple of years.

OPEC's share of the world oil market has declined even more sharply than has the total demand for oil. Put the other way, the market shares of Mexico, the North Sea operators and other non-OPEC producers have been gaining. And yet to be heard from, down the road, are China and still other large potential sources.

But the IEA bureaucrats have not been paying attention. They might read this from another "advertorial" in the four-page spread in The Wall Street Journal:

"The current financial year will be a crucial test for the Saudi economic strategy. It is generally accepted that in order to meet its commitments, the Saudis need an oil production level of at least 6.5 million barrels a day. In recent months it has dropped to 5.5 million barrels a day as a result of the oil glut and the fall in world demand.

"The 1982-83 budget, at Saudi Rials 313,400 million, showed a current account surplus of SR 128,000 million, but that was before oil production had fallen below the break-even point . . .

"Even though forecasts in the demand for oil have become notoriously unreliable, a deficit in the current account at present seems unavoidable. With an economy still overwhelmingly dependent on one product, the government is preparing with characteristic caution for a prolonged slump in the market."

As Middle East expert Kelly suggested to me, the fall in Saudi oil revenues will not be as painful for them as it would have been two or three years ago; a good deal of their capital investment for petrochemicals, for example, has been completed.

By 1983, Verleger estimates, Saudi production will move under the 5 million barrel-a-day mark.

Five million barrels a day at today's prices will net only $60 billion annually, half of what the Saudis were taking in when they were pumping over 10 million barrels a day. A loss of $60 billion -- when they've been spending that much annually just on defense ($20 billion) and development ($40 billion) -- has got to hurt.

What does it add up to? However self-serving their paid ad in The Wall Street Journal, the Saudis are closer to the facts than the IEA. Oil demand and prices have dropped dramatically, they say, and the prospects are for a weaker, not a stronger market. They suggest, even if it is overstated, a Saudi budget squeeze. More than a year ago at Daharan, Saudi oil minister Sheik Ahmed Zaki Yamani admitted that the second wave of oil price rises had killed the golden goose by encouraging the world to develop long-term alternatives.

Ihe IEA was set up after the 1973 Arab oil embargo to help generate a unified Western energy policy. Like other bureaucracies, the IEA can't be expected to wither away, even if its services are no longer so urgently needed. But at a minimum, it should quit wasting time with scare scenarios about shortages and price hikes, and focus instead on the problems that would face the West if oil prices collapse.