A key Senate subcommittee last week voted to amend the Clean Air Act in a way the nation's oil companies claim will trigger a shortage of gasoline by the summer of 1978.

The Subcommittee on Environmental Pollution, chaired by Sen. Edmund S. Muskie (D-Maine), has warned oil companies preparing to use a gasoline additive named MMT not to do so and has warned oil companies already using it that they would have six months to remove it from their gasoline.

The reason Muskie's committee has moved against MMT is the growing evidence that it fouls the catalytic converters installed in cars built in the last four model years to reduce engine exhausts, making them useless after a little as 15,000 miles of driving. Normally, converters are expected to last a minimum of 50,000 miles.

"Preliminary testing indicates that MMT causes increase in engine-out hydrocarbon emissions," General Motors Corp. told the Muskie committee. "Our tests also indicate MMT negatively influences durability and causes deterioration of our future emissions control systems."

Almost unknown outside the automotive industry, MMT is a manganese compound that has been put in gasoline in tiny traces with tetraethyl lead for 20 years to boost octane ratings. It has what is called a "synergistic" effect on lead in gasoline, meaning its presence made the lead raise octane ratings more than the lead did by itself.

When lead was ordered phased out of gasoline, a few oil companies substituted MMT for lead in unleaded gasoline. They found that it boosted octane ratings in the same way lead did, by stopping the reaction in the fuel that results in a portion of the fuel and air mixture going unburned during combustion.

No fewer than 50 U.S. refineries now add about 10 million pounds of MMT a year to about 40 per cent of the unleaded gasoline sold in the United States today. Most are small refiners who use the MMT so they don't have to lay out large sums of money to refit their refineries, but at least five big refiners use it too, according to Ethyl Corp. which makes MMT. They are Exxon, Gulf, Standard Oil Co. of California, American Oil Co. and Atlantic Richfield.

Gulf has said it will have to run as much as 8 million barrels of additional oil a year through its refineries to make as much gasoline as it makes now using MMT to boost octanes for high compression engines. Either that, Gulf says, or it will lose as much as 3 million barrels of low-octane gasoline every year because it won't burn right in today's cars.

Gulf says if every refiner now using MMT were foeced to abandon its use it would result in a loss of 60 million barrels of gasoline in one year. That's 2.5 billion gallons of gasoline, enough to trigger a nationwide shortage for the unleaded fuel that now makes up more than 20 per cent of U.S. supply.

The subcommittee does not accept the oil industry's arguments about MMT, claiming the additive costs more fuel by fouling spark plugs and exhaust control devices. But it made one concession to the oil industry: it provided that refiners now using it would get 180 days to phase it out of gasoline instead of the 90 days it originally wrote into the law.

The oil companies claim the charges that MMT fouls spark plugs and exhaust control devices are greatly exaggerated. They claim new tests by the Environmental Protection Agency will prove that, a claim that EPA says it will be happy to check out.

So far, auto tests suggest the opposite. Spark plugs have been fouled by MMT after 17,000 miles of driving. Exhaust control devices have neen plugged after as little as 15,000 miles. Newer exhaust systems that are to be installed in next year's cars are even more sensitive to the manganese additive.

What happens if MMT comes out of gasoline in six months? The oil companies predict a gasoline shortage by the summer of 1978. They also forecast a price increase of a penny of gallon in two years because it will take that much more oil to produce high octance gasoline.

The subcommittee is unimpressed by these forecasts. Staff aides say they heard the same warnings when lead was ordered out of gasoline three years ago.