The two leading European multinational oil companies have signed agreements to buy Iranian crude oil at $30 a barrel, but spokesmen for the two firms said today that Tehran also has reduced drastically the amount of oil it was making available to them.

Saudi Arabia sells comparable high-grade crude for $6 a barrel less. This difference in price appears likely to create major tension on world oil markets.

On Monday Japan permitted some of its companies to buy Iranian crude at the premium $30 rate, prompting criticism from U.S. officials hoping to hold down prices paid for Iranian oil as part of the pressure on Tehran to release the hostages at the U.S. Embassy there.

Sources in Tokyo, however, said Japan decided to approve the higher rate only after it learned that the two European companies, British Petroleum and Royal Dutch Shell, broke the line on prices. The two firms once dominated the Iranian oil fields.

British Petroleum signed a contract Wednesday for 95,000 barrels a day for nine months, compared to 195,000 barrels during the last three months of 1979. Shell will purchase 125,000 barrels a day, compared to 385,000. Both companies said they would have preferred buying at least as much as they had bought before. Iran's production was running about 3.5 million barrels a day last year.

It was not immediately clear whether the cut in daily supplies to the two companies means that Iran is reducing its production or further diversifying its clientele.Last year, Iran began selling oil to more than 40 companies, compared to the 10 customers it traditionally had.

The two major European multinationals had withdrawn their negotiators from Tehran when the Iranians were insisting on a $35 per barrel price. British oil experts expressed fear that Japanese companies would accept a subsequent offer of $30 a barrel.

The timing of the two companies' accord with the Iranians Wednesday, well after the Japanese had signed, seemed to contradict Japan's contention that it broke ranks after Shell and BP. It is possible, however, that the companies may have made informal agreements with Iran before the Japanese signed.

The companies may be able to get increased Iranian supply in supplementary agreements to refine and possibly even to market petroleum for the National Iranian Oil Co. Negotiations to that effect are underway in Tehran.

The two European companies have been the hardest hit of the seven major international oil companies by the cutbacks in Iranian production from a prerevolutionary high of 6 million barrels.

By contrast, four of the five American-based international oil majors are in much better shape because Saudi Arabia, their principal foreign source, has continued to supply them at substantially the same levels as before.