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GASOLINE ENGINE OIL VISCOSITY RECOMMENDATIONS = 10W - 30
Since 1980, the preferred viscosities for GM gasoline engines are SAE 10W-30 for the larger engines and SAE 5W-30 for the smaller engines (engines sizes defined later).
SAE 10W-40 is not included in the viscosity chart for 1984 GM gasoline engines, for several reasons, including:
^ Fuel economy with SAE 10W-30 oils is about 1% better than that with SAE 10W-40 oils. ^ SAE 10W-30 oils generally produce less piston and ring deposits than SAE 10W-40 oils. Such deposits can lead to stuck rings and increased oil consumption. The use of SAE 10W-40 engine oil in a vehicle equipped with a gasoline engine will not automatically void the General Motors Corporation New Car Limited Warranty. However, since SAE 10W-40 engine oil is not recommended for use in 1984 GMvehicles equipped with gasoline engines, if the use of such oil causes damage, the cost of the engine repair will not be covered by the Warranty. The Warranty does not cover any damage due to the use of non-recommended oils, whether SAE 10W-40 or some other viscosity. In addition, the Warranty does not cover damages caused by the use of oils that do not meet the recommended API quality or SAE viscosity grade.
need help on a 1999 Chevy K3500 pickup with a 7.4 L