It is possible to do chemically. It is far easier to make lube or diesel from it. The refinery process from crude is designed to be efficient, for the target fuel. Lube oil is not really a primary target. Also think about this. We use maybe 200-300 gallons or so of gas between oil changes of a gallon to gallon and a half of oil. We generate a million tons of gas or so to a few thousand tons of oil. Diesel or kerosene or low grade fuel oil is the best use in terms of energy efficiency and cheap to do. Also there are many industrial fuel uses for this oil, one that has not been mentioned is industrial scale heating plants. They often burn #4 fuel oil. Almost like 90wt. It has to be heated before the nozzle because it is so thick.