The "oil industry" is said to oppose enactment of President Carter's crude oil tax, and certainly that is true of Mobil Corp., the nation's third-largest oil company.

But it turns out not to be true of Exxon, the largest, nor of a number of lesser giants such as Gulf, Standard of Indiana and Continental.

Though often portrayed as monolithic the oil industry on close examination is not. Companies differ in their circumstances, and it is not unusual to find them opposing each other over the legislative fine print that can make a difference of millions of dollars in an individual company's profits.

And now that House Senate conferees are nearing their final decisions on the President's energy bill, this intramural competition is intensifying.

"It all gets back to what position your own company is in. You've got to remember we're not all alike," says one experienced oil lobbyist.

"As the days dwindle down, there is great pressure to cut and run," another oil executive says, referring to the temptation to break with the public solidarity the industry has generally maintained on the energy bill so far.

Exxon and Mobil on the crude oil tax are an example. During the American Petroleum Institute's annual convention in Houston a few weeks ago, Exxon's chairman, Clifton Garvin, called on oilmen to private meetings to work for a compromise on the crude tax that would benefit the industry.

But Mobil which along with Texaco has a reputation as a "hardliner" among the major oil companies, continues to resist the tax.

Federal energy experts say there is a fairly easy explanation for Mobil's attitude. The crude oil tax would drive up the price of oil entering U.S. refineries but it would drive up some refiners' costs more than others. Mobil has an advantage over some competitive under the present complicated oil pricing system, the federal experts say Under the Carter proposal, it would lose that advantage.

There are other examples of internal differences in the industry.

An amendment, to the energy bill offered by Sen. Floyd Haskell (D-Col) that would likely lead to a higher tariff on imported refined petroleum products is no. surprisingly, supported by companies with major stakes in domestic refining such as Ashland, Pennzoil, and Cities Service. Along with 17 other companies, they are pushing for the protectionist tariff to improve their competitive postions.

Haskell's amendment, however, is already facing quiet opposition from Continental Oil, Exxon, and other international oil companies with major foreign refining operations.

An even wider-scale contest is under way over legislative efforts to continue so-called small refiner bias, a subsidy program for small refiners under the present pricing system.

Carter's tax on crude oil would knock out the subsidy. The small refiners who benefit from the program are lobbying earnestly to save it.

Companies that are not major beneficiaries of the subsidy, such as Hawaiian Independent Refinery, Inc., and Clark Oil are trying to trim it back, or kill it altogether.

Oil insiders are also wondering what will happen to companies such as Union Oil that have large inventories of so-called old oil, which is sold under federal controls at about $5.5 a barrel, compared with the $13.50 that Carter proposed for new discoveries.

Union is pressing for a new pricing energy bill that would let old oil prices rise substantially, even if that has to be at the expense of new oil. But others in the industry - particularly independent producers - would rather see new oil prices rise at the expenses of old, if there has to be some limitation.

Natural gas, as usual, is the industry's most hard-fought issue. Many oil companies now privately favor supportin some kind of compromise between the Senate-passed bill to deregulate new gas prices, and the House bill that would set prices on new gas at $1.75 per thousand cubic feet. The House bill also extends controls to the unregulated intrastate market.

"The real hangup on gas," one top oil executive says, "is the small-producer exemption."

Major oil companies and some independent oilmen, such as Denver-based Chandler and Associates who work closely with the majors to drill for oil and gas, oppose exemptions from taxes or regulations for small producers.

Sen. Gary Hart (D-Colo.) has offered an amendment that is strongly backed by Oklahoma oilman Bob Hefner and others to exempt small producers from any price regulation. Hefner, who oilmen say is preparing to run for the Senate from Aklahoma, is also leading an effort to exempt natural gas in deep formation from regulation. Both provisions, if passed, would benefit Hefner's company.

Oilmen from major oil companies and trade associations argue against the small-producer exemptions. One says, "It doesn't matter whether your grandmother produces oil or Exxon everyone should get the same treatment."