WITH OIL PRICES falling, the time is right to step up the tax on it. But how should this be done? Unfortunately the most prominent proposal-- the tariff on imported oil--is also the least satisfactory. A far better alternative is to raise the tax on gasoline specifically. How about raising the gas tax 10 cents a gallon each April for the next three years?

The trouble with a tariff on imported oil is that it would behave just like a tariff on anything else. It would raise the profits of American producers and refiners at the expense of their customers. Of all the industries that do not need additional government assistance, oil production and refining stand pretty close to the top of the list. A tariff would raise the prices of all oil used in this country, both domestic and foreign. But it would be collected only on the oil --currently about one-third of American consumption--that is imported.

That's not a very good deal for American consumers or for American industries that compete in export markets. In the 1950s and 1960s, the United States used import quotas to keep its domestic price higher than the world level. A European scholar, Peter R. Odell, has suggested that this differential may have been as important a contribution to European economic growth as the Marshall Plan was after World War II.

It would be preferable to impose a flat excise tax on all oil, whether produced here or abroad. That's what President Reagan half-endorsed in the contingent tax plan--now moribund--in his January budget. At $5 a barrel, it would raise about $28 billion a year. Like a tariff, it would impose a competitive disadvantage on American industry, but unlike the tariff it would make a serious contribution to reducing the federal budget deficit.

A gasoline tax increase of 30 cents a gallon would raise the same amount of money. But there would be no burden on industrial fuel. Nor would it affect home heating oil. A stiff increase in the gasoline tax raises serious amounts of money with few of the undesirable economic and social side effects of a broader oil tax. The country has already demonstrated that it can accommodate large increases in gasoline prices without any undue disruption. The tax would continue to push toward conservation in an area where it can be relatively easily achieved. With the availability of smaller and more efficient cars, it becomes less painful to cut consumption. The gasoline tax can be justified either as revenue policy or energy policy. Taking both together, it's hard to think of a more logical way to raise $30 billion.