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In a previous issue we published an article entitled "Choosing Between Synthetic Lubricants and Conventional Oils." In this article, the author did an excellent job explaining some of the potential benefits of using synthetics lubricants. But are synthetic lubricants always a wise choice?
To answer this question, we must take a critical look not just at the benefits of synthetic lubricants, but also the potential pitfalls. For example, synthetic lubricants are typically made by taking small building-block molecules and joining them together, a process referred to as polymerization.
Because of the amount of work required to convert these starting ingredients into finished base oils, synthetic oils are generally more expensive than conventional mineral oils, from three to five times more for common synthetics such as polyalphaolefins (PAO) to several hundred times for highly specialized fluids such as fluorinated polymers that are used in applications requiring inherent chemical inertness.
But can the additional cost for synthetic oils be justified? The answer to this question really needs to be "it depends." For example, imagine being able to extend the oil drain interval using a high-quality PAO synthetic-based oil by a factor of six or seven simply by purchasing a product that is five times the cost.
From this perspective, it would appear that the additional cost is money well spent, particularly when you consider the additional costs associated with an oil change. But if the oil change interval is driven not by base oil degradation, but rather due to additive depletion or the buildup of certain contaminants such as water, particles or soot, then perhaps the cost of a synthetic-based oil cannot be justified.
Cost is not the only factor to consider. For example, there's the question of solvency. One of the main benefits of petroleum-based synthetic oils compared to their mineral oil cousins is the elimination of certain "undesirable" classes of molecules. Key among these is aromatics that contribute to a lower resistance to oxidation. But solvency can also be a good thing.
For example, there have been numerous studies showing that the poor solvency of certain types of petroleum-based synthetics (and for that matter, highly refined mineral oils) can result in a greater tendency to lay down varnish in high-temperature applications such as gas turbines because the oil does not have the solvency to keep the oil-wetted components clean.
Under these circumstances, the problem may be resolved by using oils with a higher natural solvency, perhaps a less highly refined mineral oil, or a nonhydrocarbon-based synthetic such as certain esters, or polyalkylene glycol (PAG) fluids which have excellent natural solvency.
Solvency also plays a role in lubricant formulation. With highly pure hydrocarbon synthetics or refined mineral oils, getting additives to dissolve in the lubricant can be a major challenge. Under these conditions, a less highly refined base stock, or a co-base stock that has greater additive solubility characteristics can be a distinct advantage.
Greater shear stability and reduced energy consumption are also often stated as advantages to synthetic lubricants. But while there is definitely truth to this statement, you must ask yourself what is causing excessive energy consumption. For example, in a worm drive application where significant energy losses can be attributed to sliding friction, the more uniform size of the molecules within synthetics lubricant can potentially allow for reduced energy consumption based on lower internal fluid friction.
But if the energy loss is not due to fluid friction but some other factor such as poor mechanical maintenance or the wrong oil level, even a pure, high-quality synthetic oil may not make a significant difference, particularly in applications such as spur gears, which are already 95 percent (or more) efficient at power transmission.
Relation to Water
Then there's the question of water. Some synthetics such as PAOs are distinctly hydrophobic - they resist water. Others, however, such as PAG and esters, not only attract water (hydrophilic), but in some instances react with water, which can lead to their downfall. Therefore, compatibility with other fluids and sealing materials should always be a concern.
So what's the answer, are synthetics really better? Instead of trying to answer this question directly, consider each lubrication point as requiring a lubricant that has a defined series of physical (viscosity, viscosity index, etc.) and chemical (oxidation resistance, active wear protection, etc.) performance properties.
These need to be decided based on machine type and application, as well as other factors such as ambient environment and accessibility for oil changes. Once this has been completed, the last step is to chose the product - whether that product be mineral or synthetic - that meets each performance criteria.
Once you take this pragmatic approach, the choice between mineral and synthetic becomes clear. In some cases, the only way to achieve the necessary performance properties will be to choose the appropriate synthetic oil. But in other cases, an acceptable level of performance can be achieved by selecting a good-quality, well-formulated mineral-based lubricant, avoiding some of the pitfalls outlined above.