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An engine oil's viscosity at operating temperature has fallen steadily over 10 000 km although no fuel dilution is found. The likely cause is

  • A.shearing of the viscosity index improver
  • B.oxidation of the base oil
  • C.loss of the pour point depressant
  • D.exhaustion of the anti-foam additive

Answer: A

The long polymer chains are broken by repeated passage through the bearings and the improver's thickening effect is progressively lost. Oxidation would raise the viscosity and the acid value together rather than lowering it.

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