Contamination Control Lubricants Lubrication

Research And Test When Switching Lubricants

EP Editorial Staff | February 9, 2023

Opposite. When synthetic oils or greases are involved, the chances of incompatibility are much higher than mineral-oil-based lubricants.

Changing lube and/or grease vendors requires a pause to thoroughly evaluate comparability and compatibility factors.

By Mark Barnes, PhD CMRP, Des-Case Corp.

With global supply chain issues continuing to affect lubricant supplies, many companies are becoming creative at ensuring they have enough lubricants to operate equipment safely and reliably. One such approach is to seek alternate vendors who may have shorter lead times and more immediate availability. While this can work, asset managers need to be cautious before they proceed.

At face value, it may seem like a simple exercise to switch from one vendor of AW68 hydraulic fluids to another or from one multi-purpose grease to a similarly labelled product. In doing so, look at the comparability and compatibility, particularly if your intent is to simply add the new oil or grease into an existing system without extensive flushing or cleaning.

Comparability factors

In the case of lubricants, comparability simply means, if you switch from one lubricant to another will they perform in a similar way in similar applications? Comparability of different lubricants can be inferred from looking at the product datasheets (PDS) of the two lubricants and comparing key physical chemical properties such as viscosity, viscosity index, load-carrying capability, and oxidation stability.

Under most normal applications, oils and greases that possess similar physical and chemical properties might be expected to
perform similarly under “typical” operating conditions. 

Compatibility factors

Lubricant compatibility, however, is completely different. Compatibility refers to how a mixture of two nominal comparable lubricants will perform under operating conditions. For lubricating oils, consider the compatibility of the two main components of a lubricant—the base oil and the additive package. For mineral oils that are derived from refined crude oil, base oil compatibility is highly likely since the base-oil molecules are similar in nature. However, care must be exercised when mixing mineral oils with synthetic oils or mixing two different synthetic fluids with different base-oil chemistries.

Most synthetic industrial oils are PAO (poly-alpha-olefin) based. As such, they are derived from long-chain hydrocarbons, chemically similar to the hydrocarbon molecules found in mineral oils. As such, incompatibility between mineral oils and PAO synthetics is uncommon, though differences in solvency can result in deposit formations. However, maximum care must be exercised when switching from a hydrocarbon fluid—either mineral or PAO—to an API Group V synthetic such as an ester or polyalkylene glycol (PAG). Because of the different base-oil chemistries, blending fluids from API Group V with hydrocarbon-based fluids can result in an adverse chemical reaction that can lead to sludge, varnish, and gelation. This issue is particularly acute with PAG fluids, which should never be mixed with hydrocarbon fluids without an extensive flushing process.

In some cases, flushing may entail more than draining and refilling the system several times and may require the use of an intermediate flushing fluid, either with a very low additive treat rate, e.g., an R&O (rust and oxidation inhibited) type fluid, or with a base oil chemistry that is compatible with the two fluids to be exchanged, for example, using polyol ester to switch between a PAG fluid and a hydrocarbon base oil.

With the exception of Group V fluids, most incompatibility between lubricating oils results from the additive composition, not from the base oils. Even fluids that are nominally comparable, e.g., two AW68 mineral oils may be incompatible when blended in certain ratios. Additive incompatibility usually manifests itself in the form of impaired physical performance characteristics such as foaming tendency or demulsibility (inability to shed water).

Grease considerations

With lubricating greases, consider base-oil and additive compatibility along with thickener compatibility. Grease-thickener chemical composition ranges widely from simple lithium soaps to polyurea, clay, and complex soap thickeners of calcium and aluminum. Mixing greases of different thickener types can have disastrous consequences, from hardening of the grease in the bearing housing to extreme thinning, causing the grease to leak out through the shaft seal.

Fig. 1: Typical Grease Compatibility Chart. Compatibility charts should never be used to determine whether it is safe to mix two different greases.

Fig. 1: Typical Grease Compatibility Chart. Compatibility charts should never be used to determine whether it is safe to mix two different greases.

Thickener incompatibility is a well-known phenomenon, giving rise to a number of so-called “compatibility charts” such as the one shown in Figure 1. Compatibility charts should never be used to determine whether it is safe to mix two different greases. Even two greases that are considered “compatible” in one chart may not necessarily be so when mixed under operating loads, temperatures, and ambient stressing conditions.

When mixing must happen

In some circumstances, such as when changing lubricant vendors, mixing different lubricants is unavoidable. When this is anticipated, it’s always prudent to test their compatibility by blending the two products in the ratio in which they might be mixed and testing key physical and chemical properties to determine if any significant changes have occurred.

In the case of lubricating oils, compatibility testing entails mixing the two lubricants in a blend ratio of 10:90, 50:50, and 90:10. Tests, such as foaming tendency, air entrainment, demulsibility, and oxidation stability should be conducted on all three blends, as well as samples of the two oils to be mixed. Any significant variation between the pure new oil test results and the blended mixtures should be taken as a sign of incompatibility, meaning that oil sumps, lines, and all oil-wetted components should be thoroughly drained and flushed when switching to the new oil type.

The most common effects of grease incompatibility are changes in grease consistency, i.e., thickening or thinning of the mixture. To test for grease compatibility, the two greases should be blended in 25:75 and 75:25 ratios and tests performed to identify changes in thickness and oil bleed rate after the mixtures are subjected to shear stress through a grease “worker.”

Just like oils, any evidence of incompatibility in mixed greases should be considered a significant issue, requiring disassembly of the system followed by thorough cleaning and removal of the old grease.

Fortunately, many lubricant suppliers have tested the impact of compatibility with common competitor products. As such, asking your lubricant vendor for this data is often a good starting point. However, make sure you’re clear that you’re seeking information (data) on compatibility, not just comparability.

Whether you’re looking for more readily available lubricants, seeking a more competitive price, or switching lubricant vendors due to corporate contractual issues, do not under-estimate the importance of lubricant compatibility. Always ask for supporting data and, where none is available, think carefully about submitting samples to a specialized lab capable of conducting the proper compatibility tests. EP

Mark Barnes, PhD CMRP, is Senior Vice President, Global Business Development, at Des-Case Corp., Goodlettsville, TN ( He has 25 years of experience in lubrication management, oil analysis, and contamination control.


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