An engine test procedure was developed to measure blowby rates with a modified European diesel test engine. With monograde oils, the rate correlates with viscosity. The higher the engine blowby level the less the engine sensitivity to lubricant viscosity. The differences in blowby control observed with multigraded oils appear to correlate with their viscosities at temperatures well above 210°F (98.9°C). Other factors such as temporary shear loss may play a part as indicated by qualitative observations. The same general trends were also confirmed in an unmodified multicylinder gasoline engine.
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