Antipollution experts from major oil companies flew into Trinidad and Tobago today and sharply downgraded initial estimates of the oil spill from Thursday's supertanker collision off this southeastern Caribbean republic.

While many questions still remain about the first crash of fully loaded tankers of more than 200,000 dead-weight tons, interviews with oil officials, agents for the vessels involved, and government officials of Trinidad and Tobago have disclosed the following uncontested facts:

Initial fears and reports of a doomsday oil spill apparently have proved unfounded. Although the two vessels together carried nearly 3.2 million barrels of oil - nearly one-fifth the daily petroleum consumption of the United States - neither had lost more than 10 percent of its cargo by late today.

The 1,066-foot Atlantic Empress - reported by police and local Coast Guard officials here Saturday to have exploded and sunk - was still afloat today. Two oceangoing tugs had managed to get a cable to the bow of the burning vessel. They were towing it to the north where prevailing currents could sweep its still-leaking cargo out into the Atlantic.

Firemen who Saturday boarded the blazing 1,086 foot Aegean Captain extinguished flames billowing from a gash in the tanker's bow. Although the ship was still leaking oil, it also was being towed north, away from the palm-fringed beaches and coral reefs of this flower-strewn Caribbean island.

Twenty-six crewmem from the Atlantic Empress and one from the Aegean Captain are still missing.Of the remaining 44 seamen involved, only three were still hospitalized. Two crewmen from theAtlantic Empress have been transferred to a Trinidad hospital with facial burns and the captain of the Atlantic Empress, Pascalis Hatzpetrou, was flown to a special treatment center in Galveston, Tex., with burns on 50 percent of his body.

A spokesman for the ship's owners said Hatzpetrou was the last man off his ship after the collision. By the time he left, the spokesman said, the tanker's superstructure was "white hot" and clothing was igniting as crewmen sought to lower the lifeboat.

Exactly how and why the collision occurred remained a mystery, although some intriguing clues surfaced.

Victor Cockburn, deputy commissioner of customs for Trinidad and Tobago, said official government inquiries posed by him to agents and crews of both vessels disclosed that the collision occurred at 7:15 p.m. Thursday, 18 miles northeast off Tobago's northern tip.

Residents of the community of Bayside saw the fire at sea and reported it to authorities here, Cockburn said. At about the same time, he said, an Eastern Airlines jet passing overhead heard a Mayday call from the ships and relayed it to the Trinidad airport.

Cockburn, who said he is compiling evidence for both Lloyds of London and the British Board of Trade, said heavy rains at the time made radar images on both ships "fuzzy." They first saw each other, he said, when they were just 600 feet apart.

Supertankers of this size - each longer than three football fields and higher than a nine-story building - require six miles between them to turn safely, Cockburn said.

Although their exact speeds at the time of the crash have yet to be determined, Cockburn said, the average speed of such vessels in open waters would approach 18 to 20 knots, or 21 to 23 miles per hour.

"You must fully understand the momentum of a vessel of that tonnage," he said. "At that speed it would require 15 miles to stop."

Piskopianos Christos, an officer of the Aegean Captain, said his ship was eastbound from the Netherlands Antilles when it suddenly encountered the Atlantic Empress.

International rules of the sea require both vessels in such a situation to turn left to avoid a collision.

The Aegean Captaid did so, Christos alleged, but the Atlanti Empress was late in swerving. No independent sourch could confirm his account. The Aegean Captain's starboard bow, he said, then struck the Empress amid-ship on the starboard.

Reflecting after the accident, Christos questioned why the Atlanti Empress - which reportedly was bound from the Persian Gulf to Beaumont, Tex. - was heading south, which he thought would be off course. Tankers, however, are not required to follow set courses.

Although Christos' version of the tankers' precollision maneuvers is at least partly borne out by gashes in the vessels, no one could be found to confirm his story.

George Papoulas, Trinidad agent for the Atlantic Empress, when asked about the vessel's position, said only, "Why the hell would he be heading south?" and explained that in emergency situations ships "may not always be able to follow the rules of the road."

Also unclear is the destination of the Aegean Captain's cargo. Mobil Oil Co. has acknowledged ownership of the nearly 2 million barrels of oil aboard the was bound for Beaumont, Tex... But David B. Archer, Trinidad agent for the Liberian-registered Aegean Captain, said he did not know exactly who owned the oil his vessel supposedly was carrying to Singapore.

While the Aegean Captain's cargo had been reported since the collision as "naphtha" or "Venezuelan crudes," Archer confirmed today that it too is Arabian crude oil loaded from transshipment tanks on the Netherlands Antilles island of Bonaire.

Just why a tanker should come halfway around the world to a point off the Venezuelan coast to pick up Arabian oil for Singapore in the Far East no one could say. But one oil engineer and an official of Trinidad and Tobago suggested separately that it was not bound for Singapore but for South Africa and that the Caribbean trip was only a "laundering" maneuver to sidestep trade embargoes.

As for the oil on the water, a Mobil spokesman said it was now split into two slicks - one slick 15 miles by 2 miles around the Aegean Captain and another 10 miles by 2 miles around the Atlantic Empress. The vessels were last reported 45 to 50 miles northeast of here.

Unlike the persistant oil spills of winter tanker wrecks like that of the Argo Merchant in 1977, the oil spills here appeared to be getting smaller with time rather than larger. Spray planes, and detergent-spraying boats brought here by Mobil and other cooperating oil companies were helping that effort, Mobile officials said, but a larger factor was the nature of the particular oil type.

"People think of an oil spill as being only one thing," a petroleum engineer said today, "but actually there are many different kinds depending on the grade of oil."

Light Arabian crude, like that carried on both vessels here, is "very volatile...almost 25 percent gasoline." Much of it was burned, he said, but what has not - like this Caribbean crisis itself - is simply evaporating.