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"When you are changing lubricants, if both lubes are acceptable for a particular application, can it be assumed that there is a strong likelihood that they would be compatible? If they are incompatible, wouldn’t the severity be limited? Would incompatibility be more of a concern if you were switching oil types?"
Lubricant formulations are more sophisticated than you might think. The specifics of their formulations are frequently the reason incompatibility becomes an issue, even with two oils of the same type. Synthetics often have incompatibility concerns when mixed with mineral-based oils, with a few potential exceptions, such as polyalphaolefins (PAOs) and esters. However, even if two synthetic base oils (or a synthetic and a mineral-based oil) are compatible and acceptable for a certain application, they can still be incompatible due to their additive packages. So regardless of the base oil compatibility, the additives associated with the base oil must be considered.
Generally, an additive has three main functions: to enhance the base oil properties, to suppress the base oil properties or to impart new properties to the base oil. An additive package is developed specifically for the particular type of base oil, since the base oil will provide a different set of properties for the lubrication tasks. That said, extra caution should be taken when dealing with lubricants that have different base oils.
Keep in mind that lubricant suppliers can achieve similar performance characteristics in a variety of ways. If a changeover is made between lubricant brands but the lube types are the same, there could still be incompatibility with the additives. Most additive incompatibility is due to some initial chemical reaction that results in further degradation or neutralization. This may lead to the additive function being lost or altered as well as an undesirable byproduct formation.
Compatibility should also be considered beyond simply the two lubricants in question. For instance, even if a new synthetic lubricant performs the lubrication function effectively, it may be introducing an incompatibility concern with the materials within the system, such as the seals. Therefore, the seal material and any potential incompatibilities with certain base oil types should be taken into account before selecting the oil for the machine.
In conclusion, never assume two lubricants are compatible, regardless of the applicability. While they may be compatible, they should be processed with due diligence before they are allowed to mix. If you are not confident of their compatibility, perform a flush using best practices to ensure every effort has been made to remove all remnants of the previous oil.