In recent years, several new types of base oils have become available on a global basis. These include high-quality mineral oils, specialty PAOs and vegetable oil-derived oils as well as recently introduced gas-to-liquid (GTL) base oils. Re-refined base oils are becoming more available also - are they as good as virgin base stocks? The currently-available collection of commercial base stocks feature a broad continuum of quality and, depending on their composition, level of purity and viscometric properties, they are classified by the American Petroleum Institute as Group I, II, III, IV or V. There are now more different types of lubricant base stocks than ever before, which raises the question once again: Can one predict how base oil properties affect performance in finished lubricants and how formulations can be optimized to meet targeted specifications? Two important properties in a finished lubricant, viscosity and volatility, are dominated by the base oil blend, but the base oils can also strongly impact more subtle blending strategies and tactics. In order to better understand how different types of base stocks behave in finished products, it will be helpful for the formulator to not only understand the chemistry of the various types of base stocks, but also how different types of base stocks are manufactured. There is no perfect single base stock -- each type has advantages and limitations, depending on the final application.