Managing the Risk of Mixing Lubricating Oils
By Arnold L. Shugarman

There has been some feedback on an article I had written about synthetic oils and the effects of mixing oils, brands and base stocks. So today we went to one of the industries experts who wrote an article for POA Magazine to find his opinion on the matter.

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.

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 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.

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