Engine Oil: Notes
WARNING: This page is about a different car, the 2002 Saturn LW200, 2002 Saturn L200, and 2002 Saturn L100. However, it is still accessible from the selected car via links, so may be relevant.
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.