As the quality of motor oil has improved over the last 15 years, so has the quality of petroleum-based (mineral oil) heat transfer fluids.

The old way to refine oils - solvent de-waxing - has given way to hydrotreating, which is more expensive but produces oils that have almost no residual sulfur compounds or asphalt molecules, according to a recent “TipSheet” written by Paratherm Corp.’s Jim Oetinger, the director of technology. With no impurities present to start the fluid degradation process, the new oils have much longer service lives and can operate at higher temperatures, he says.

How can you tell the difference? Color.

Oetinger notes that impurities give oil a yellow color - the darker and less transparent, the more impurities are present.

Yellow may be cheaper to buy, but it is not cheaper to own. When shopping for mineral-oil based fluid, ask for a sample from each potential supplier. Pour each fluid into a clear-glass jar and compare. If it is the color of beer, it is old technology, he cautions.

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