This is actually a somewhat complicated issue. Can it be used indefinitely? No. But with proper management, including good filtration, a lubricating oil can last many times longer than a typical OEM recommended service interval.
|The table above demonstrates the relative life of a component based on its cleanliness level. As an example, if we had a rolling element bearing operating at a cleanliness level of 23/21/18, it would only achieve 30% of its relative useful life. If that relative useful life is 10 years based on operational rated loads and speeds, the bearing would only last 3 years before a potential replacement is needed. Conversely, if the same bearing were operating at a cleanliness level of 16/14/11, 100% of its relative useful life could be achieved, representing a 3-4 fold life extension.|
There are several factors that determine the useful service life for a lubricating oil—including the oil type, quality, reservoir volume, equipment type, and operating conditions. The operating conditions that most heavily influence oil life are temperature, contamination levels, and the amount of residue from previously used oil commonly referred to as sludge.
The most common contaminants that curtail oil life are water, air, and metal wear debris particles. For example, a lubricating oil that is contaminated with water and copper particles can oxidize up to 18 times faster than one that is clean and dry. Particles, and to some extent water, can easily be filtered from the oil allowing it to achieve a longer service life. If vacuum dehydration is used, the water levels can be reduced to the point of insignificance. Of course to achieve the benefits, the decontamination would need to be performed regularly to keep levels consistently low.
The other factor that can be mitigated with filtration is sludge. As oil degrades, it creates byproducts that compromise the stability of new oil additions by consuming the antioxidants in the new oil. This residue can be filtered from the oil by using special depth media or electrostatic filtration.
Even with the best of treatment, a lubricating oil will eventually fail due to additive loss unless the oil can be “sweetened” by a partial drain and refilling with new oil or even by the addition of a concentrated additive package, which is a rare practice. Any attempts to reclaim used oil by filtering or otherwise decontaminating it, should be verified with rigorous testing of the oil to guarantee its fitness for use. The best policy in most cases is to continuously control contamination, manage oil condition and use quality oil analysis to determine when the oil has reached the end of its useful service life.