Today’s high performance lubricants are specifically formulated with a carefully selected balance of performance additives and base stocks to match the lubrication requirements of the equipment in which they are used. When lubricants are mixed, this balance is often upset. Mechanical problems leading to shorter equipment life can occur, sometimes catastrophically.

Modern lubricants are sophisticated products, formulated to meet the demanding lubrication requirements of modern equipment. The old saying, “oil is oil” no longer applies. Mixing lubricants is fraught with danger - to your equipment, to your business and to your wallet. When in doubt, don’t mix different lubricants. If it occurs accidentally, address the problem immediately. Don’t be afraid to bring in an expert, whether it is the lubricant manufacturer, the additive supplier or an independent consultant to your site. Your response to a situation where different lubricants are mixed will depend on the products in the mixture, the end-use application, the relative concentrations of products and the total volume involved.

See Table 1

In its mildest form, mixing different lubricants may lead to a degradation of lubricant performance. Mixing the same API grades of synthetic passenger car motor oil and mineral oil-based engine oil won’t damage the engine, but you will lose the performance features you expect from the synthetic. At the other end of the spectrum, adding typical turbine oil to an antiwear hydraulic oil in a hydraulic pump could spell disaster. Deposits may form that could increase wear and plug filters.

Why Formulation is Important
To understand why some mixed oils are OK but others are not, one must understand how modern lubricants are formulated. Most performance lubricants are a blend of base stocks and additives. The base stock is the oily portion of the lubricant, chosen for the physical and chemical properties needed in the final blend. Base stocks, in most industrial lubricants, are selected based on the requirements for viscosity, oxidation stability, fire-resistance, biodegradability and water miscibility in the final product. They carry the load in hydrodynamic lubrication, remove heat and debris from friction and wear and help seal out contaminants.

Most lubricants are formulated with mineral base stocks that are severely refined, low-wax, heavy distillate fractions of crude oil. They are relatively low cost, generally good solvents for most additives, available in a wide viscosity range and compatible with a number of seal materials. Synthetic base stocks are made by chemical manufacturers to impart special qualities to the finished oil. Polyalphaolefin (PAO), organic esters, glycols and phosphate esters are examples of synthetics that are used to meet specific needs. Synthetics are used where the value of their special functional properties, oxidation stability, fire-resistance, etc., outweigh their cost.

Lubricants made with synthetic base stocks should not be mixed with products made with mineral oil, even if they are designed for the same application. The limited exceptions include some PAO and ester-based products. Even then, compatibility is often concentration-dependent. Deposits may form because of additive incompatibility or seal compatibility may be compromised.

Additives impart special performance features to the finished oil. The choice of additives and the balance among them differentiate an antiwear hydraulic oil from a turbine oil, for example. Some additives affect the physical properties of the finished lubricant. Others change the lubricant’s chemical properties or are added for cosmetic purposes.

Lubricant Incompatibility
Some lubricants are incompatible because of differences in additive chemistry that lead to undesirable chemical reactions. If these oils are mixed, insoluble material may form and then deposit onto sensitive machine surfaces. For a hydraulic fluid, this could lead to lubricant starvation, valve failure or increased wear.

A second form of lubricant incompatibility is more insidious because no visible changes occur when the products are mixed. The problem appears only after the mixture is used in a piece of equipment that consequently fails or loses performance. For example, hydraulic/tractor fluid that is contaminated by motor oil can lead to brake chatter and failure in farm equipment. Optimum performance requires carefully balanced frictional and antiwear properties in the finished product that are upset when the lubricants are mixed.

Some incompatible lubricant mixtures may also affect synthetic rubber seals. Lubricants are formulated to be neutral to seals or cause them to swell slightly. Too much seal swell, seal shrinkage or chemical deterioration may occur with some combinations of lubricants. Engine oils formulated with certain types of dispersants attack fluorocarbon seals. Lubricants contaminated by products containing ester base stocks may swell seals unacceptably. EP gear oils are known to deteriorate silicone seals.

Lubricant incompatibility is a chemistry problem. It has nothing to do with the manufacturers of the oil; two oils made by the same manufacturer may be incompatible. The most common cause of lubricant incompatibility that results in the formation of harmful solids is the reaction of an acidic component in one oil with a basic component in another. The reaction is accelerated by water and heat.

Oils containing acidic rust inhibitors are incompatible with oils containing basic rust inhibitors. When the two oils are mixed, especially when some water is present, a solid forms in the oil that reacts further with the oil to form a grease-like insoluble substance. This can clog filters, form deposits that interfere with lubrication and interfere with demulsibility (water-oil separation).

One way to look at potential lubricant incompatibility is to classify lubricants as acidic or basic. Table 2 is not meant to be totally inclusive and some exceptions may occur. Different lubricant manufacturers may use different additive chemistries to accomplish the same function, so caution is warranted. Check with your manufacturer to be sure.

Lubricants that require good demulsibility (water separation) should never be mixed with lubricants that contain dispersants or high concentrations of detergents. Small amounts of oil with good emulsion characteristics will destroy the water shedding properties of a highly demulsible lubricant. Rarely can the demulsibility be restored with additive supplements.

Lubricants formulated with non-zinc antiwear and antioxidant additives such as railroad engine oils and ashless or low-ash gas engine oils will cause engine damage if they are contaminated with lubricants containing zinc additives.

How to Avoid Lubricant Mixtures
By now, you should have enough information to realize that consciously mixing lubricants should not be undertaken without a thorough knowledge of the lubricant chemistry and the intended application. If in doubt, don’t do it.

But problems can occur all along the supply chain from the lubricant manufacturer to the end-use equipment. The manufacturer puts the wrong label on the drum. The marketer pumps oil into the wrong storage tank.

The maintenance mechanic uses the wrong container to add makeup oil to the hydraulic fluid sump.

Incidents of lubricant mixing can occur without any evidence of negligence. A trucking firm changed the manufacturing source of heavy-duty diesel engine oil and upgraded to the newest API performance category. Within a few thousand miles, the oil was black and all oil condition-monitoring signs indicated that the oil needed to be changed. After investigating the problem with the bad oil, the lube engineer concluded that the replacement oil had such strong dispersancy that it was cleaning all the engine deposits generated from the use of the original lubricant. The remedy was to change oil at a shorter interval until the engines were clean, at which point the trucking firm realized the full benefits of the new oil.

Accidental mixing of oils can be minimized by sound practices - clearly labeling containers, checking manifests on oil shipments against the delivered products, supervising the unloading of bulk oil and segregating oils that are known to be incompatible. Avoid using common hoses, funnels and containers to transfer lubricants from different families. For example, the same lubricant handling equipment can be used for different engine oils, but never transfer turbine oil with equipment used to transfer engine oils. Less than 0.2 percent engine oil can form emulsions in turbine oil. Before changing lubricant suppliers or accepting a new/improved product from your current supplier, obtain assurance that the new product is compatible with the old one. Ask what tests were conducted to demonstrate the product compatibility.

Fighting an Oil Mixing Problem
When an oil mixing problem occurs, certain prudent steps should be followed. For bulk oil, empty the tank before adding the new product. If products are incompatible, it may be necessary to flush the tank with diesel or base oil. If oil mixing is suspected, isolate the system. Thoroughly drain the oil from equipment that contains a suspected mixture. When seeking advice on how to handle the contaminated oil, provide the following information:

  • Identity of each lubricant in the mixture. Include name of manufacturer and brand name.

  • Volume of each component in the mixture.

  • Lubrication application.

  • Other lubricated equipment, often less severe applications, where the oil mixture might be used.

  • Where the mixed oil is located - one bulk tank or 10 gear boxes.

  • Ability to store oil mixture until final disposition can be determined.

With this information, intelligent decisions can be made quickly to avoid equipment operating problems or damage, minimize downtime and identify the best way to dispose of the oil mixture.

You can conduct some simple tests to confirm an oil mixing problem even without access to a formal lubricant laboratory. Heat an oil mixture or two oils you want to test for compatibility and examine for clarity. If the mixture is cloudy, the oils are not compatible. To check further, add a small amount of water, mix thoroughly and continue heating. Allow the mixture to sit at room temperature for several hours. If a solid forms in the oil, they are incompatible.

To test for demulsibility (water-shedding properties) in a mixture, mix equal parts of warm oil and water in a bottle, shake thoroughly and allow the mixture to settle. If both oils have good demulsibility, the oil and water layers should separate cleanly with little or no emulsion between the layers. Even if these tests indicate that the oils are compatible, check with the manufacturer to see what other tests should be run before using the mixture in your equipment.

Before mixing lubricants or using lubricants that have been mixed, check with the oil manufacturer for the best course of action. Most importantly, look for ways to avoid the situation in the first place.