GM Cutlass RWD 1970-1987 Repair Guide

Fuel Recommendations


See Figures 1 and 2

Most gasoline engines from 1975-87 are designed to use only unleaded gasoline. Unleaded gasoline must be used for proper emission control system operation. Its use will also minimize spark plug fouling and extend engine oil life.

The use of a fuel too low in octane (a measurement of anti-knock quality) will result in spark knock. Since many factors such as altitude, terrain, air temperature and humidity affect operating efficiency, knocking may result even though the recommended fuel is being used. If persistent knocking occurs, it may be necessary to switch to a higher grade of fuel. Continuous or heavy knocking may result in engine damage.

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Fig. Fig. 1: Recommended oil viscosities (Gas)

Your engine's fuel requirement can change with time, mainly due to carbon buildup, which will in turn change the compression ratio. If your engine pings, knocks, or runs on, switch to a higher grade of fuel. Sometimes just changing brands will cure the problem. If it becomes necessary to retard the timing from the specification, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage, in addition to increasing the engine temperature.

All diesel engines in temperatures above 20°F (-7°C), are to use Number 2 diesel fuel. In temperatures below 20°F (-7°C) Number 1 diesel fuel is recommended. In some areas of the country, a combination of Number 1 and Number 2 diesel fuel is available and is recommended for winter use.

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Fig. Fig. 2: Recommended oil viscosities (Diesel)

The reason that Number 2 diesel fuel is not recommended for cold weather operation is, at temperatures below 5°F (-15°C), paraffin wax flakes form that thicken the fuel and block the fuel filter.


If you plan to drive your car outside the United States or Canada, there is a possibility that fuels will be too low in anti-knock quality and could produce engine damage. Send to Oldsmobile's customer service department the (VIN) Vehicle Identification Number, compression ratio of your engine and the countries in which you plan to operate.They should be able to provide you details of adjustments or modifications that can be made to your engine. It is also wise to consult with local authorities upon arrival in a foreign country to determine the best fuels available.