What Goes Into an Oil: Base Fluids (Basestocks)

There are two main components that any motor oil is made of. There is a base fluid (sometimes called a basestock) and the additive package. The base fluid typically makes up the bulk of the oil. Additive chemicals are then added to enhance the positive qualities of the basestock and to overcome whatever negative qualities there may be.

There are two main types of basestocks, petroleum and synthetic. Petroleum basestocks are a purified form of crude oil and have been used as the base for automotive lubricants since motor oils were first being developed.

Synthetic basestocks, on the other hand, are chemically engineered in a lab specifically for the purpose of lubrication. They are engineered from pure compounds that contain no contaminants which must be removed via purification. Synthetic basestocks have been around since the early 1900's but were not widely used in automotive type applications until the 70's.

Petroleum Basestocks: Desirable Qualities

As indicated above, petroleum basestocks are refined from crude oil that has been recovered from natural underground "storage areas". Once the oil is recovered, it must be run through a series of purification steps to improve the following desirable lubrication qualities:

1. Viscosity Index
   
   A measure of an oil's ability to maintain it's viscosity over a wide temperature range. The higher the number, the less change in viscosity with a change in temperature. Better oils will generally have higher viscosity indexes.
   
   
2. Low Temperature Performance
   
   The better an oil will flow at low temperatures, the better its low temperature performance. Better low temperature performance provides more immediate engine protection at start-up in cold weather climates.
   
   
3. High Temperature Performance
   
   How well does an oil hold up under extremely hot conditions. Will it burn off easily? Will it allow metal to metal contact under hot conditions as a result of viscosity loss? Obviously, better oils will hold up more effectively under extreme heat.
   
   
4. Oxidation Resistance
   
   Oxidation occurs when oxygen reacts with the components of an oil to form sludge and other engine deposits. Oxidation leads to increased oil viscosity making the engine work harder to pump the oil through its system. An oil should be able to resist oxidation.

Petroleum Basestocks: The Refining Process

In order to enhance the above qualities for the final lubricant base oil, crude oil is passed through a series of purification steps. The series of steps will be something closely resembling the following:

1. Desalting
   
   Removal of salt contaminants from the crude oil to make the rest of the refining process easier.
   
   
2. Partial Vaporization
   
   The crude oil is heated within a vaporization chamber which collects portions of the crude that have differing boiling points. Lubricating basestocks are the components with the highest boiling point with the exception of asphaltic materials.
   
   
3. Vacuum Distillation
   
   Process by which lubricating basestocks are separated into fractions of differing molecular weights, and, therefore, differing viscosity ranges.
   
   
4. Solvent Extraction
   
   Solvents are added to each fraction obtained from the distillation process and the mixture is allowed to settle into a phase containing aromatic compounds and a phase containing non- aromatic compounds. The aromatic compounds are extracted from the basestock before the next step in the refining process.
   
   Up to 80% off the aromatic contaminants are removed through this method. This greatly improves thermal and oxidative stability and raises the viscosity index of the stock considerably.
   
   
5. Dewaxing
   
   Wax is removed to improve low temperature fluidity. In cold temperatures wax contaminants will crystalize making the lubricant thick and difficult to pump.
   
   Methyl ethyl ketone (MEK) is added to the lubricant basestock and the oil is cooled to just below the intended pour point of the basestock. All wax crystals that form are removed via filtration.
   
   NOTE: The pour point of an oil is often referred to in its technical specifications and basically refers to the lowest temperature at which an oil will still pour (it's actually slightly more complicated than that, but pour points will be discussed in another chapter).
   
   
6. Hydrofinishing or Clay Treatment
   
   This is an optional component of the refining process reserved for more premium petroleum basestocks. Hydrofinishing uses a catalyst bed through which hydrogen and heated oil are passed. As these components pass through the bed, unstable components such as sulfur and nitrogen are removed. Clay treatment uses a different method to achieve a similar outcome.
   
   Both of these refining processes improve oxidation stability, thermal stability and color of the lubricant basestock.
   
   
7. Hydrotreating
   
   In some cases a more severe method is used in addition to regular hydrofinishing. Hydrotreating involves putting the lubricant basestock through extremely high temperature and pressure extremes in the presence of a catalyst.
   
   This will convert any remaining aromatic hydrocarbon contaminants into usable nonaromatic hydrocarbon molecules. The resulting hydrocarbon molecules are much more stable, and the resulting basestock is very pure with very few contaminants.
   
   This process can be used in place of solvent extraction of aromatics and/or in addition to solvent extraction. It is much more effective, achieving about 99% removal of aromatic contaminants as opposed to only about 80% for solvent extraction. Only super-premium petroleum basestocks will be manufactured using this method.

A Note of Importance - Crude oil comes from many sources and has a wide range of quality levels and contamination levels. The refining process above can only do so much. As a result, petroleum basestocks will have a wide range of quality levels.

To minimize these quality differences lubricant companies must exercise tremendous care in selecting crude oil stocks. In addition, the refining process must be done under the strictest of quality control measures.

As a result, those companies that exercise this care will charge more for their oil - they simply have to. So, if you are going to use a petroleum lubricant, keep in mind that you generally get what you pay for. Although you're paying somewhat for the brand name, in most cases there is a reason that brand name oils are priced higher - they're of higher quality.

Just because you see the API starburst on the bottle doesn't mean it's a "quality" oil - it only means that the oil in that container meets the absolute minimum specifications in order to adequately protect your engine. Just be careful what you use to protect your "baby".

Additional Info via the Links Below:

page 1 - current : Intro - Petroleum Basestocks
page 2 : Pseudo-Synthetic Basestocks
page 3 : Synthetic Basestocks
page 4 : So What Does AMSOIL Use?