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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.

Intake valve sticking, created by the accumulation of deposits on the valve stems, caused cold starting problems and engine failures at low temperatures (below + 5°C). Sticking was promoted by the high olefin content in gasoline (over 25% FIA). Sticking did not occur if gasoline contained no detergent additives. Some additives even led to sticking while some solved the problem. Engine oils containing polymeric additives promoted sticking. Clearances of intake valve stems and stem seals were probably factors affecting sticking. Tests were made using cars on the road on an urban driving cycle.