GM Full-Size Trucks 1988-1998 Repair Information

Fuel and Oil Recommendations



See Figures 1, 2, 3 and 4

The Society of Automotive Engineers (SAE) grade number indicates the viscosity of the engine oil; its resistance to flow at a given temperature. The lower the SAE grade number, the lighter the oil. For example, the mono-grade oils begin with SAE 5 weight, which is a thin light oil, and continue in viscosity up to SAE 80 or 90 weight, which are heavy gear lubricants. These oils are also known as "straight weight", meaning they are of a single viscosity, and do not vary with engine temperature.

Multi-viscosity oils offer the important advantage of being adaptable to temperature extremes. These oils have designations such as 10W-40, 20W-50, etc. The 10W-40 means that in winter (the "W" in the designation) the oil acts like a thin 10 weight oil, allowing the engine to spin easily when cold and offering rapid lubrication. Once the engine has warmed up, however, the oil acts like a straight 40 weight, maintaining good lubrication and protection for the engine's internal components. A 20W-50 oil would therefore be slightly heavier than and not as ideal in cold weather as the 10W-40, but would offer better protection at higher rpm and temperatures because when warm it acts like a 50 weight oil. Whichever oil viscosity you choose when changing the oil, make sure you are anticipating the temperatures your engine will be operating in until the oil is changed again. Refer to the oil viscosity chart for oil recommendations according to temperature.

The American Petroleum Institute (API) designation indicates the classification of engine oil used under certain given operating conditions. Only oils designated for use "Service SG, SH" or greater should be used. Oils of the SG, SH or its superseding oil type perform a variety of functions inside the engine in addition to the basic function as a lubricant. Through a balanced system of metallic detergents and polymeric dispersants, the oil prevents the formation of high and low temperature deposits and also keeps sludge and particles of dirt in suspension. Acids, particularly sulfuric acid, as well as other by-products of combustion, are neutralized. Both the SAE grade number and the APE designation can be found on top of the oil can.

Diesel engines also require SG, SH or greater engine oil. In addition, the oil must qualify for a CC or greater rating. The API has a number of different diesel engine ratings, including CB, CC, and CD. Any of these other oils are fine as long as the designation CC appears on the container label along with them. Do not use oil labeled only SG, SH or only CC. Both designations must always appear together.

For recommended oil viscosities, refer to the chart. Note that 10W-30 and 10W-40 grade oils are not recommended for sustained high speed driving when the temperature rises above the indicated limit.

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Fig. Fig. 1: Gasoline engine oil viscosity chart

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Fig. Fig. 2: Look for the API oil identification label when choosing your engine oil

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Fig. Fig. 3: Diesel engine oil viscosity chart

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Fig. Fig. 4: Recommended lubricants

Synthetic Oil

There are many excellent synthetic and fuel-efficient oils currently available that can provide better gas mileage, longer service life, and in some cases better engine protection. These benefits do not come without a few hitches, however; the main one being the price of synthetic oils, which in most cases is more expensive per quart of conventional oil.

Synthetic oil is not for every truck and every type of driving, so you should consider your engine's condition and your type of driving. Also, check your truck's warranty conditions regarding the use of synthetic oils.


Gasoline Engines

It is important to use fuel of the proper octane rating in your truck. Octane rating is based on the quantity of anti-knock compounds added to the fuel and it determines the speed at which the gas will burn. The lower the octane rating, the faster it burns. The higher the octane, the slower the fuel will burn and a greater percentage of compounds in the fuel prevent spark ping (knock), detonation and preignition (dieseling).

As the temperature of the engine increases, the air/fuel mixture exhibits a tendency to ignite before the spark plug is fired. If fuel of an octane rating too low for the engine is used, this will allow combustion to occur before the piston has completed its compression stroke, thereby creating a very high pressure very rapidly.

Fuel of the proper octane rating, for the compression ratio and ignition timing of your truck, will slow the combustion process sufficiently to allow the spark plug enough time to ignite the mixture completely and smoothly. Many non-catalyst models are designed to run on regular fuel. The use of some super-premium fuel is no substitution for a properly tuned and maintained engine. Chances are that if your engine exhibits any signs of spark ping, detonation or pre-ignition when using regular fuel, the ignition timing should be checked against specifications or the cylinder head should be removed for decarbonizing.

Vehicles equipped with catalytic converters must use UNLEADED GASOLINE ONLY. Use of unleaded fuel shortened the life of spark plugs, exhaust systems and EGR valves and can damage the catalytic converter. Most converter equipped models are designed to operate using unleaded gasoline with a minimum rating of 87 octane. Use of unleaded gas with octane ratings lower than 87 can cause persistent spark knock which could lead to engine damage.

Light spark knock may be noticed when accelerating or driving up hills. The slight knocking may be considered normal (with 87 octane) because the maximum fuel economy is obtained under condition of occasional light spark knock. Gasoline with an octane rating higher than 87 may be used, but it is not necessary (in most cases) for proper operation.

If spark knock is constant, when using 87 octane, at cruising speeds on level ground, ignition timing adjustment may be required.

Your engine's fuel requirement can change with time, mainly due to carbon buildup, which changes 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 specifications, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage and will increase the engine temperature.

Diesel Engines

Diesel engines require the use of diesel fuel. At no time should gasoline be substituted. Two grades of diesel fuel are manufactured, No. 1 and No. 2, although No. 2 grade is generally more available. Better fuel economy results from the use of No. 2 grade fuel. In some northern parts of the U.S. and in most parts of Canada, No. 1 grade fuel is available in the winter or a winterized blend of No. 2 grade is supplied in winter months. When the temperature falls below 20°F (-7°C), No. 1 grade or winterized No. 2 grade fuel are the only fuels that can be used. Cold temperatures cause unwinterized No. 2 to thicken (it actually gels), blocking the fuel lines and preventing the engine from running.

Do not use home heating oil in your truck.
Do not use ether or starting assist fluids in your truck.
Do not use any fuel additives recommended for use in gasoline engines.

It is normal that the engine noise level is louder during the warm-up period in winter. It is also normal that whitish/blue smoke may be emitted from the exhaust after starting and during warm-up. The amount of smoke depends upon the outside temperature.


If you plan to drive your truck 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. It is wise to consult with local authorities upon arrival in a foreign country to determine the best fuels available.