Viscosity is the most important single property of lube oil. In used oil analysis, the test is performed to determine whether the oil has thickened or thinned excessively. Abnormal results indicate that some operational or maintenance defect exists that should be corrected. Fuel Dilution is the most common cause of oil thinning in diesel engines. The results of fuel dilution are reported as percentage by weight or volume.
Water can contaminate diesel engine crankcases as a product of combustion or due to coolant leaks.
Water contamination is very seldom found in diesel engine oil samples. Engines that operate for long periods of time create enough heat to evaporate contaminating water. However, evidence of ethylene glycol in crankcase oil indicates that coolant is leaking into the lube system and warrants corrective action. Total Solids , the quantity of insoluble material in the oil, is a good general indicator of lubricant contamination level. Fuel soot normally accounts for most of the material measured in the solids test.