Process for upgrading lubricating oil stock

A process for preparing a stabilized lubricating oil resistant to oxidation and sludge formation upon exposure to a highly oxidative environment which comprises a first step of contacting a high boiling hydrocarbon fraction lubricating oil stock with elemental sulfur in the presence of a catalyst material selected from the group consisting of alumina, silica, aluminosilicate, a metal of Groups II-A, II-B, VI-B or VIII of the Periodic Table of Elements, an oxide of a metal of Groups II-A, II-B, VI-B or VIII, a sulfide of a metal of Groups II-A, II-B, VI-B or VIII, clay, silica combined with an oxide of a metal of Groups II-A, III-A, IV-B or V-B. and combinations thereof in a flow reactor at a reaction temperature of from about 150.degree. C to about 225.degree. C, a reaction pressure of from about 0 psig to about 500 psig, a liquid hourly space velocity of from about 0.5 hr.sup.-1 to about 20 hr.sup.-1, said elemental sulfur being added in an amount of about 0.025 to about 0.2 percent by weight of said oil stock, and a second step of contacting the product of the first step with hydrogen in the presence of alumina impregnated with at least about 10 weight percent of MoO.sub.3 and at least about 2.5 weight percent of CoO, said impregnated alumina having at least 50 percent of its pores with a pore diameter of 50 Angstrom Units or more, at a reaction temperature of from about 80.degree. C to about 190.degree. C, a reaction pressure of from about 100 psig to about 300 psig and a liquid hourly space velocity of from about 0.1 hr.sup.-1 to about 10 hr.sup.-1, said hydrogen in the second step being circulated at from about 100 scf/bbl to about 1500 scf/bbl.