What Are Base Stocks?
Motor oils start with a base oil mixed with various additives. These base oils often account for approximately 80 to 90 percent of the volume, and are therefore the backbone of oil. Everyone knows that some oils are petroleum-based and some synthetic, while others are labeled semi-synthetic. What does all this mean? Well, not as much as it used to, because the lines are now blurred in the case of synthetic oils.
For our purposes, petroleum oils are the most basic and least expensive oils on the market. They are created from refined crude oil and offer good properties, though they are generally not as heat resistant as semi-synthetics or full synthetics. On the other end of the spectrum are synthetic oils. A synthetic oil has been chemically reacted to create the desired properties. Semi-synthetics are a blend of the two base stocks.
The API groups oils into five major categories, each with different properties and production methods:
Group I: Solvent frozen mineral oil. This is the least processed of all oils on the market today and is typically used in nonautomotive applications, though some of it may find its way into low-cost motor oils.
Group II: Hydro-processed and refined mineral oil. This is the most common of all petroleum oils and is the standard component of most petroleum-based automotive and motorcycle engine oils.
Group III (now called synthetic): The oils start as standard Group I oils and are processed to remove impurities, resulting in a more heat-stable compound than possible as a standard Group I or II oil. Some examples are Castrol Syntec automotive oil and Motorex Top Speed. These are the lowest cost synthetics to produce, and generally do not perform as well as Group IV or V oils.
Group IV: Polyalphaolefin, commonly called PAOs. These are the most common of the full synthetic oils, and usually offer big improvements in heat and overall stability when compared to Group III oils. They are produced in mass quantities and are reasonably inexpensive for full-synthetic oils. Since they are wax-free they offer high viscosity indexes (low temperature pour point) and often require little or no viscosity modifiers. Examples include Amsoil and Motorex Power Synt.
Group V: Esters. These oils start their life as plant or animal bases called fatty acids. They are then converted via a chemical reaction into esters or diesters which are then used as base stocks. Esters are polar, which means they act like a magnet and actually cling to metals. This supposedly offers much better protection on metal-to-metal surfaces than conventional PAOs, which do not have this polar effect. These base stock oils also act as a good solvent inside the engine, translating into cleaner operation. Esters are the most expensive to produce, and oils manufactured with them usually cost much more. Due to this higher cost, many companies only fortify their oils with esters. Some examples are Bel-Ray EXS, Torco MPZ Synthetic and Maxum 4 Extra. Motul 300V, however, uses 100 percent ester as its base oil, and is one of the more expensive oils.
The grouping of these oils is the source of much controversy. One topic that has been debated is what can be labeled a "full synthetic oil." In 1999, Mobil brought a complaint against Castrol for changing the base oil in its Syntec product. They had used a Group IV PAO, but had changed to a Group III base oil. Mobil contended that Group III oils were not really "synthetic oil" and should not be labeled as such. After many expert opinions were heard, the National Advertising Division of the Better Business Bureau sided with Castrol and said that Group III oils could be labeled synthetic. Since that time there has been a lot of growth in this product type due to its low cost and similar performance to traditional synthetics. Many traditionalists still argue that Group III oils are not true synthetic oils.
