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"What are the effects of mixing synthetic gear oil with mineral gear oil?"
The effect depends on the types of synthetic bases that were mixed with mineral bases. Polyalphaolefin and diester synthetic bases can be mixed with mineral oil bases, which is done regularly to create “blend” products.
Polyalkylene glycol (PAG) bases should not be mixed with any of the others unless specialized barrier fluids are used to minimize the incompatibility. When PAGs are mixed into other lubricants, you typically will get strong negative reactions (producing sludge and tacky residue) that require extra effort to flush, clean and correct.
Even if the base oils are compatible, there is the prospect that the additives used to create necessary performance properties could conflict, producing lost lubricant effectiveness.
It is advisable to perform filterability, oxidation stability, air release and demulsibility testing prior to mixing lubricant intentionally.
Remember, 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 anti-wear hydraulic oil in a hydraulic pump could spell disaster. Deposits may form that could increase wear and plug filters.
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.