Excellent fuel quality or fuel detergents are required to reduce intake valve deposits in gasoline engines. However, engine oils also have a considerable effect on the deposits.In engine tests the cleanest valves were found with synthetic multigrade or mineral single grade oils, both containing no VI improvers. Popular SAE 10W-40 and 5W-50 oils formed more deposits because they contain VI improvers. Deposit weight correlated directly with oil shear stability which is a measure of the VI improver quality. Both engine oil and gasoline have to be chosen correctly for a clean intake system.