THE REAGAN administration's oil deal with Mexico is a mistake. Under this agreement, the U.S. government is to buy oil for its strategic reserve directly from the Mexican government. This country is apparently abandoning its previous salutary policy against direct government-to-government oil trading. Why? So far, there's no clear explanation.

In the past, the U.S. government has bought a lot of foreign oil, but always through intermediaries -- the international oil companies. The oil companies have provided a valuable buffer against the political and diplomatic interests that crowd in on any transaction between two governments. Unlike the U.S. government, Exxon, Texaco and the rest don't have an immigration policy to enforce along the Rio Grande. They aren't involved in disputes over water rights. They aren't responsible for drug enforcement programs. They don't have to arbitrate quar rels over imported Mexican tomatoes that compete with the Florida crop. The oil companies have nothing to offer for oil but mere money, and money is all that they ever pay.

It's never that simple when governments trade with each other. Between two immediate neighbors like the United States and Mexico there are always dozens of issues under discussion, and the treat ment of one inevitably affects the outcome of others. If an oil company thinks that a seller's price is too high, it can simply walk away from the offer. But the United States couldn't break off its pur chases from Mexico without creating a political inci dent of some magnitude. It's not very likely that this oil will be bought at a strictly market price -- particularly if prices begin to move. In a falling mar ket, the U.S. government--unlike the oil companies --would have to show a measure of concern for the economic stability of its neighbor. This agreement is a strange choice for an administration that keeps talking earnestly about the necessity to hold future spending to a minimum.

If there should be another oil crisis like the last two, with prices shooting upward, the sellers of oil would once again have great leverage. This adminis tration's energy policy, here as in other cases, seems to be based mainly on the cheery hope that there won't be any more oil crises and shortages.

The amounts of oil in this agreement are not very large, by the standards of the oil business. After the turn of the year, the volume will fall to about 1 percent of U.S. imports. The gains, in terms of the strategic re serve, will be only modest. But the political implications are large, and the precedent is an unwise one.