UNFORTUNATELY, the Strategic Petroleum Reserve is rapidly turning into this year's great example of budgetary sleight-of-hand. Just about everybody agrees that the reserve is essential. But the $3.9 billion that President Reagan has requested for it next year has attracted the attention of the increasingly desperate budget-cutters. There's a sort of a contest running at the Capitol to see who can come up with the cleverest scheme to buy and store the oil without seeming to pay for it -- or at least without having it turn up in the federal budget.

The Senate Budget Committee succeeded in getting its total spending figure lower than Mr. Reagan's by the simple expedient of dropping the money for the petroleum reserve altogether. The committee suggests, vaguely, "alternative" financing. The House Budget Committee, in a similar exercise, deleted $1.5 billion of the money for the reserve with a hopeful reference to "private" funding.

Private funding won't work. In an emergency, the private owners' interest would be to keep holding the oil as the price rose. If private financing merely means selling oil reserve bonds on the private market, like the bonds of other federal agencies, it's hardly worth doing. Since the impact on the economy is the same in both cases, it makes little difference whether Congress calls it spending or borrowing.

If Congress is determined to get the Strategic Petroleum Reserve off the budget, there is one clean and straightforward way to do it. The reserve specifically benefits the users of oil, and there's nothing wrong in making the users pay for the reserve directly. One obvious solution is a conventional tax on oil imports, but the Reagan administration won't countenance a conventional tax. So why not an unconventional tax -- payable in oil? Why not a requirement that, for every 20 barrels of oil imported into this country, the importer must contribute one barrel, as an insurance premium, to the reserve?

A year ago, the reserve was so far behind schedule that questions of financing had little practical importance. But things have changed since then, and by last month the underground caverns were being filled at the tremendous rate of 400,000 barrels a day. The reserve now equals 28 days' imports, and it's beginning to represent a genuinely useful cushion. The present period, with soft prices and a slight oversupply worldwide, is ideal for filling it. Congress would be wanton to let the quarrel over the budget interfere with the strategic reserve.