A system for producing an American Petroleum Institute Standards Group III Base Stock from vacuum gas oil, by injecting hydrogen, heating, saturating the mixture through hydrogen reactors connected in series with a liquid hourly space velocity (LHSV)−1 of from 0.5 to 2.5, forming a saturated heated base oil, and coproduct. The system fractionates the saturated heated base oil to while simultaneously refluxing a cooled fuel oil fraction forming an American Petroleum Institute Standards Group III Base Stock with less than 0.03% sulfur, with greater than 90% saturates and a viscosity index greater than 120 as defined by ASTM D-2270, a viscosity from 2 to 10 centistokes as defined by ASTM D-445 a boiling range from 600 degrees F. to 1050 degrees F., and a cold crank viscosity (CCS) between 1200 and 5000 centipoise at −25 degrees C. and as defined by ASTM D-5293.