insolubles - test for contaminants in used lubricating oils, under conditions prescribed by test method ASTM D 893. The oil is first diluted with pentane, causing the oil to lose its solvency for certain oxidation resins, and also causing the precipitation of such extraneous materials as dirt, soot, and wear metals. These contaminants are called pentane insolubles. The pentane insolubles may then be treated with toluene, which dissolves the oxidation resins (benzene was formerly used). The remaining solids are called toluene insolubles. The difference in weight between the pentane insolubles and the toluene insolubles is called insoluble resins. Testing for grease insolubles is described in ASTM D 128.