All about oil
Close-up on synthetic motor oils
The two categories and four sub-categories of mineral oil base stocks offer far less variation than their synthetic counterparts. In some cases, the differences among synthetic base stocks sometimes cause them to act more like antagonists than conspirators.
Esters are compounds made by reacting carboxylic acids with an alcohol. The result is a bunch of little “ball bearings” of lubricant. Esters are a ubiquitous lot, showing up as everything from polyesters in leisure suits to the plasticizers used in polyvinylchloride (PVC). The wide variety is due to differing methods of manufacture. There are several chemical configurations for the base acid, as there are for the alcohol with which it reacts. The manufacturer can juggle the properties of the end lubricant by selecting the two ingredients that best suit his goal.
Esters can be made from raw materials grown on the farm, although polyol esters are between 10 and 30 percent petroleum, while diesters run between 60 and 80 percent. The reason for using petroleum as a starting point is one of economics; for the time being, it is less expensive to start with petroleum than with anything else.
Synthetic hydrocarbons are constructed by starting with a petrochemical compound and then building on it, creating long hydrocarbon chains. Synthesized hydrocarbons resemble mineral oils to the extent that neither has oxygen in the molecule, while esters, with 10 percent oxygen, resemble animal or vegetable base oils.
One such synthesized hydrocarbon is polyalphaolefin. It starts with a simple ethylene molecule, which then goes through a “cloning” process (actually, alpha olefin preparation, oligomerization, and then hydrogen stabilization) so that long strings of identical molecules bind together in chains.
The overriding similarity among the synthetics is that they are monomolecular. That is, they are basically one molecule, whether standing alone or connected in a long chain with other identical molecules. The difference between a synthesized oil molecule and a refined petroleum molecule is like the difference between a laser and a light bulb. The refined petroleum molecule provides lubrication by throwing a bunch of diverse elements at the problem and hoping one will work. The synthesized oil molecule is constructed specifically to provide the best possible lubrication under a defined condition. This makes their behavior very predictable, and is the reason some synthetics exhibit naturally high Viscosity Indexes. Add the powerful V.I. polymers included in most synthetics, and you have an oil with an incredible range of operational temperatures.
Monomolecularity is also the reason synthetics are less volatile than mineral oils. Volatility is the measure of a lubricant's tendency to boil away at high temperatures. Mineral oils, with their lumpy blend of light and heavy fractions, tend to boil away the lighter fractions when heated. This leads to viscosity change and oil thickening. If taken to an extreme, this boiling away could eventually leave you with something resembling asphalt — not the ideal lubricant.
It is also significant that synthetics are natural detergents. Although almost all synthetic motor oils have a detergent package in them, the package can be tailored more carefully due to the inherent detergent action of the synthetic. Also, since synthetics do not break down the way mineral oils do, the detergent/dispersant package is not used up fighting the degradation of the base stock, as can happen in mineral oils. This means extra life for the detergent engine protection.