Engine Oil: Notes

At the refinery, crude oil is separated by heating and distilling it into gases, light fuels, solvents and lubricating-oil base stocks. After being further processed, the base stocks are selectively blended and fortified with additives to produce an engine oil with the desired properties.

Engine oil, then, is a sort of prescription. It contains different ingredients to do different things. Some of these things are done by the base oil, the rest by the additives.