Hmmm. That’s a tough question. The answer is not as simple as “yes” or “no”. A better question would be: Is synthetic oil the best choice for this application? All types of synthetic base oils can be the best choice for certain situations. The trick is identifying those situations where they make sense or provide value.
There are plenty of potential benefits to using synthetic oil vs. mineral oil, but that doesn’t mean that synthetics are necessarily better. In order to get value from using a higher-priced synthetic oil, you must ensure that you are utilizing the potential improved performance of those products; and to make those determinations, you need to understand the conditions that allow synthetics to provide that value.
To more fully understand this issue, first consider the major advantages of common types of synthetic base oils and then identify the conditions for which these advantages become benefits.
For the sake of brevity, I will not discuss all synthetics, but rather focus on the most common ones – PAOs (polyalphaolefins), PAGs (polyalkaline glycol), diester and polyol ester.
PAOs, often called synthetic hydrocarbons, are probably the most common type of synthetic base oil used today. They are moderately priced, provide excellent performance and have few negative attributes.
PAO base oil is actually similar to mineral oil. The advantage comes from the fact that it is built, rather than extracted and modified, making it more pure. Practically all of the oil molecules are the same shape and size and are completely saturated.
The potential benefits of PAOs are improved oxidative and thermal stability, excellent demulsibility and hydrolytic stability, a high VI, and very low pour point. Most of the properties make PAOs a good selection for temperature extremes – both high operating temperatures and low start-up temperatures. In my opinion, those are the conditions that favor PAO selection. Typical applications for PAOs are engine oils, gear oils and compressor oils.
The negative attributes of PAOs are the price and poor solubility. The low inherent solubility of PAOs creates problems for formulators when it comes to dissolving additives. Likewise, PAOs cannot suspend potential varnish-forming degradation by-products, although they are less prone to create such material.
For the most part, this issue of solubility can be addressed through the addition of other base oils such as diester. The cost issue is really about whether or not you actually get value by utilizing the performance.
Polyalkaline glycols (PAGs or PGs)
PAG base oils have several unique properties that allow them to work very well in certain applications. In general, they have excellent oxidative and thermal stability, very high VI, excellent film strength and an extremely low tendency to leave deposits on machine surfaces. The low deposit-forming tendency is really due to two properties – the oil’s ability to dissolve deposits and the fact that the oil burns clean. So when they are exposed to a very hot surface or subjected to micro-dieseling by entrained air, PAGs are less likely to leave residue that will form deposits. PAGs may also be the only type of base oil with significantly lower fluid friction, which may allow for energy savings. The other unique property of PAGs is the ability to absorb a great deal of water and maintain lubricity.
There are actually two different types of PAGs – one demulisifies and the other absorbs water. The latter can be very useful if you have a compressor that cannot be stopped that is continually contaminated with large amounts of water. The most common applications for PAGs are compressors and critical gearing applications.
The negatives of PAGs are their very high cost and the potential to be somewhat hydrolytically unstable.
Dibasic Acid Ester (Diester)
The properties of diester are somewhat similar to that of PGs. It has excellent oxidative and thermal stability, very high VI and excellent solubility. This excellent solubility makes it a good choice for reciprocating compressors, where valve deposits can be a huge problem. Another common application for diester is in synthetic engine oil. It is often used as an additive with PAO basestocks to provide the necessary solvency for the engine oil’s large additive package. As a side effect, the synthetic engine oil will have excellent detergency. The negative attributes of diester are the high price and poor hydrolytic stability.
Polyol ester basestocks have several excellent performance properties, including thermal stability, super-high VI and fire resistance. Of the base oils mentioned in this column, it is probably the best choice for very high-temperature applications. The two most common applications for polyol ester are fire-resistant hydraulic fluids and jet engine oils. They can be used in engine oils and compressor applications, as well. The negative attributes are the same as those for diester.
Go for ‘Right’ Quality
There are many applications for which synthetic oils provide solutions to tough operating conditions. I have mentioned a few of those here, but there are others. To me, it is not so important to use the “best” quality lubricant in every application, but rather to use the “right” quality.
Many people waste money on expensive products that for a number of reasons don’t improve reliability or anything else. One other important thing to remember is that I am discussing the properties of base oils, not finished lubricants. It is quite possible for a finished lubricant using a mineral basestock to offer better performance than a similar product utilizing a synthetic.
So, back to the original question, is synthetic oil better? The answer is yes … and no … or maybe. You’ll have to decide.