London — With oil prices on the brink of diving in coming months, OPEC officials warn that cheaper prices will hasten the day when oil runs out altogether. ''Oil is a finite commodity,'' says an official at the secretariat of the Organization of Petroleum Exporting Countries in Vienna. ''Once it has gone, it cannot be replaced - and when world recession ends, you in the West will start using it up even faster.''
Those who agree generally predict an end of easily recoverable oil within some 20 to 30 years. But a growing chorus of other expert voices disagrees that the world is going to run out of oil at all.
''It just isn't going to happen,'' says a seasoned oil analyst in the City of London. ''The more you use, the more there is.''
In other words, the more necessary oil is to keep the world's economic engines turning, the more strenuous the efforts will be to find it, recover it, and stretch it out by developing alternatives and substitutes.
Those efforts have begun to slacken as oil prices fall and the costs of new fields and substitutes become less economical. But the experts see them picking up again later.
''The point is,'' says a respected oil analyst here, ''that when demand for oil goes back up, and the world recession ends, countries won't forget the lessons of the 1973 and 1979 price hikes.''
''Many conservation measures are here to stay,'' the analyst continues. ''Peugeot, British Leyland, and Volkswagen are all working on cars that deliver 100 miles to the gallon: A Peugeot 305 with a new engine has just traveled from the Pyrenees to Paris on a single gallon.
''People aren't going to pull out their double-glazing or their attic insulation. And if more oil is needed, money will be spent to dig into known fields as yet not even explored at all - as well as to jump back into liquefying coal, developing solar energy, and all the rest.''
Another London oilman likened the world's use of crude oil to cream on the top of a bottle of milk.
''People have been living off the cream,'' he said, ''and now they are crying hunger when they have to drink the milk itself. That 'milk' is more than enough to see us through for a long time - provided, of course, that we consumers don't suddenly go back to our bad habits and waste oil. I don't think we will.''
He defined the ''milk'' as including so-called ''frontier'' oil (lying in remote, cold, and deep places); coal liquefied with methanol and pumped through pipelines; and the so-called ''wet'' parts of natural gas (propane and butane), to substitute for oil in petrochemicals.
Shell Oil Company places world coal reserves at 720 billion tons of coal equivalent (t.c.e.), which it estimates to be the equivalent of 3.5 trillion barrels of oil. The World Energy Conference estimates the technically and economically recoverable reserves to be 663 billion, which at current rates of production will last for 239 years.
An official at Shell Oil in London laughed. ''How long will the oil last? It's like asking how long is a piece of string.
''Fifteen years ago experts were saying that oil would run out in the year 2000, or maybe 20 years later. Today they are saying oil will last well into the 21st century.''
What alarms many analysts, however, is the prospect of a drop in oil prices that goes too far, too fast.
We need five years to develop frontier oil and liquefied gas and all the rest ,'' says an oil company executive. ''Drop the price so fast that we lose money, and no one else will do the exploring and development.''
The 21-nation International Energy Agency in Paris was set up by major industrial nations to warn against dependency on imported oil when high growth rates return. It warns of trouble ahead.
Its World Energy Outlook, published in late 1982, estimated proven reserves of conventional oil at 90 billion tons (650 billion barrels), ''or about 29 years of production at 1980 levels.''
Since only 40 percent of an oil field is recoverable through normal methods, the IEA went on to say that ''ultimately recoverable resources . . . are about two to three times as large'' - possibly somewhat less, but possibly much more.
''If the value of oil rises to the world,'' says Dr. Paul Stevens, lecturer in oil economics at the University of Surrey in Britain, ''then more of it will be recovered.''
Almost 56 percent of proven reserves are in the Middle East, and some Saudi Arabian fields have not been tapped at all. North America, including Mexico, has almost 12 percent, and the Soviet Union almost 11 percent, according to the IEA.
IEA officials stress that outside North America and the North Sea, oil is in politically unstable areas.
Dr. Stevens and Rotterdam economist Paul Odell (who believes that oil will last much longer than most people imagine) say market forces will stretch supplies. Mr. Odell warns against too fast a drop in prices, saying it will be destabilizing.
Some 600 sedimentary basins have been discovered worldwide, and about one-third have not yet been thoroughly explored. Less than half of those that have been explored contained oil, but there are clearly many opportunities remaining. Many of the untapped fields are deep offshore or in polar regions. They will be extremely expensive to exploit - but if the need is there, Stevens and Odell argue, the money will be spent.
The IEA warns, ''If the world continues to use more crude oil at the average growth rate which has prevailed in recent years, present proven reserves could be exhausted in less than 25 years.''
Dr. Stevens shakes his head. ''What about new technology we don't even know about yet?'' he asks. ''What about tertiary recovery on fields where only primary and second recovery methods have so far been used? What about discoveries of new fields?''
Forecasting how long oil might last is such a hazardous business that a number of organizations contacted for this article said they had given it up.
''Too much guesswork involved,'' said an official at Britain's Department of Energy in London. ''To put it bluntly,'' said the president of Royal Dutch Shell , L.C. van Wackem, in The Hague late last year, ''we simply don't know what will happen - not even between now and the year-end.''
On the size of proven reserves, the IEA figure of 650 billion barrels is matched by a Shell Statistical Review figure of 660 billion barrels as of the end of 1981 and by a British Petroleum Company Ltd. estimate of 678.2 billion barrels.
But no one can tell for certain how much is ultimately recoverable. Much depends on what experts see as the demand for the rest of the century and beyond. There is general agreement that demand is about to lift as the current world recession moves into economic growth again.
The IEA, using low- and high-demand scenarios, sees world demand standing at between 48 and 50 million barrels per day (b.p.d.) by 1985, rising to between 50 and 56 million b.p.d. by 1990 and possibly going as high as 74 million b.p.d. by the year 2000.
It warns that world oil supplies, however, will lag behind by 1990, and could be 21 million b.p.d. in arrears by 2000.
''Nonsense,'' says one major oil company spokesman here. ''The world just isn't going to be using 74 million b.p.d. of oil by the year 2000.''