The Reagan administration is weighing whether to ease nearly 3-year-old sanctions barring U. S. oil companies from doing business in Libya, an administration official said Monday. While emphasizing that President Reagan has made no final decision, the official said the administration is concerned that continuing the prohibition actually could end up benefiting Libyan strongman Moammar Gadhafi. The official discussed the administration's thinking on grounds that he not be named publicly. Since mid-1986, several U.S. oil companies have been barred from doing business directly in Libya, but they have kept their assets, including equipment, there under an agreement in which Gadhafi recognized the firms' 49 percent share in the state-owned oil company.