Petroleum-based mineral oils function very well as lubricants in probably 90 percent of industrial applications. They are cost-effective and provide a reasonable service life if used properly but have some limitations, depending upon the specific type of base stock used, the refining technology, the type and level of additives blended, and the operating conditions encountered. The main service difficulties within mineral oils are:
1. The presence of waxes, which can result in poor flow properties at low temperature.
2. Poor oxidation stability at continuously high temperatures, which can lead to sludge and acid buildup.
3. The significant change in viscosity as the temperature changes, which can cause the base oil to thin excessively at high temperature.
4. A practical maximum high-temperature application limit of about 125 degrees C (250 degrees F) above which the base oil oxidizes very rapidly. It is desired to keep mineral oil-based lubricants within the operating range of 40 to 65 degrees C (100 to 150 degrees F).
Synthetic base oils are expensive because of the processing involved in creating these pure chemical base oils. Their use must justify the additional cost. There should be a financial benefit to using them.
With regard to their chemical purity, think of the analogy of a container of balls. Mineral oil would be like having the container filled with many different balls of different shapes and sizes, such as footballs, baseballs, tennis balls, ping-pong balls, soccer balls, golf balls, etc. Mineral oils contain thousands, if not millions, of different chemical structures (molecules). A synthetic oil would be the equivalent of having the container filled with just one type of ball (tennis balls). Every structure in the container of synthetic oil is almost identical to the structure beside it.
The two main advantages of synthetic oils are their ability to outperform mineral oils at high operating temperatures (above 185 degrees F) and at low operating temperatures (below 0 degrees F). There are other potential advantages, too.
Depending on the type of synthetic, other advantages of synthetic lubricants (beyond the high- and low-temperature advantage) may include: