GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Fuel and Engine Oil Recommendations



See Figure 1

When adding oil to the crankcase or changing the oil or filter, it is important that oil of an equal quality to original equipment be used in your vehicle. The use of inferior oils may void the warranty, damage your engine, or both.

The Society of Automotive Engineers (SAE) grade number of oil indicates the viscosity of the oil (its ability to lubricate at a given temperature). The lower the SAE number, the lighter the oil; the lower the viscosity, the easier it is to crank the engine in cold weather but the less the oil will lubricate and protect the engine at high temperatures. This number is marked on every oil container.

Oil viscosities should be chosen from those oils recommended for the lowest anticipated temperatures during the oil change interval. Multigrade oils have been developed because of the need for an oil that embodies both good lubrication at high temperatures and easy cranking in cold weather. All oils are thick at low temperatures and thin out as the temperature rises. Basically, a multigrade oil is thinner at lower temperatures and thicker at high temperatures relative to straight weight oils. For example, a 10W40 oil (the W stands for winter) exhibits the characteristics of a 10 weight (SAE 10) oil when the vehicle is first started and the oil is cold. Its lighter weight allows it to travel to the lubricating surfaces quicker and offer less resistance to starter motor cranking than say, a straight 30 weight (SAE 30) oil. But after the engine reaches operating temperature, the 10W40 oil begins acting like straight 40 weight (SAE 40) oil, its heavier weight providing greater lubrication with less chance of foaming than a straight 30 weight oil would at that temperature.

After extensive testing of 10W40 oils, General Motors recently concluded that they should be replaced by oils that offer less of a viscosity range. Note that 10W30 or 15W40 oils are preferred for use by GM on 1984 and later models, as shown in the viscosity chart that accompanies this section of this information.

Single grade (straight weight) oils such as SAE 30 are more satisfactory than multi-viscosity oils for highway driving in diesel engines.

The American Petroleum Institute (API ) designation, also found on the oil container, indicates the classification of engine oil used under certain given operating conditions. Only oils designated for use Service SE , or SF heavy duty detergent should be used in your Buick, Olds or Pontiac. For 1984 and later models, only SF oils are approved by GM. Oils of the SE and SF type perform many functions inside the engine besides their basic function of lubrication. Through a balanced system of metallic detergents and polymeric dispersants, the oil prevents high and low temperature deposits and also keeps sludge and dirt particles in suspension. Acids, particularly sulfuric acid, as well as other by-products of engine combustion are neutralized by the oil. If these acids are allowed to concentrate, they can cause corrosion and rapid wear of the internal engine parts.

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Fig. Fig. 1: Gasoline engine oil viscosity chart.

Non-detergent or straight mineral oils should not be used in your GM engine.


See Figure 2

Diesel engines require different engine oil from those used in gasoline engines. Besides performing the same functions as gasoline engine oil, a diesel engine oil must also deal with increased engine heat and the diesel blow-by gases, which create sulfuric acid, a highly corrosive compound.

Under the American Petroleum Institute (API) classifications, gasoline engine oil codes begin with an S , and diesel engine oil codes begin with a C . This first letter designation is followed by a second letter code which explains what type of service (heavy, moderate, light) the oil is meant for. For example, the top of a typical oil can will include: API SERVICES SC, SD, SE, CA, CB, CC. This means the oil in the can is a good, moderate duty engine oil when used in a diesel engine.

It should be noted here that the further down the alphabet the second letter of the API classification is, the greater the oil's protective qualities are (CD is the severest duty diesel engine oil, CA is the lightest duty oil, etc.) The same is true for gasoline engine oil classifications (SG is the severest duty gasoline engine oil, SA is the lightest duty oil, etc.).

Many diesel manufacturers recommend an oil with both gasoline and diesel engine API classifications. Consult the owner's manual for specifications.

The top of the oil can will also contain an SAE (Society of Automotive Engineers) designation, which gives the oil's viscosity. A typical designation will be: SAE 10W30, which means the oil is a winter viscosity oil, meaning it will flow and give protection at low temperatures.

On the diesel engine, oil viscosity is critical, because the diesel is much harder to start (due to its higher compression) than a gasoline engine. Obviously, if you fill the crankcase with a very heavy oil during the winter (SAE 20W50, for example) the starter is going to require a lot of current from the battery to turn the engine. And, since batteries don't function well in cold weather in the first place, you may find yourself stranded some morning. Consult the owner's manual for recommended oil specifications for the climate you live in.

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Fig. Fig. 2: Diesel oil viscosity chart.


There are excellent synthetic and fuel efficient oils available that, under the right circumstances, can help provide better fuel mileage and better engine protection. However, these advantages come at a price, which can be three or four times the price per quart of conventional motor oils.

Before pouring any synthetic oils into your vehicle's engine, you should consider the condition of the engine and the type of driving you do. Also, check the vehicle's warranty conditions regarding the use of synthetics.

Generally, it is best to avoid the use of synthetic oil in both brand new and older, high mileage engines. New engines require a proper break-in, and the synthetics are so slippery that they can prevent this; most manufacturers recommend that you wait at least 5,000 miles before switching to a synthetic oil. Conversely, older engines are looser and tend to use more oil; synthetics will slip past worn parts more readily than regular oil, and will be used up faster. If your vehicle already leaks and/or uses oil (due to worn parts and bad seals or gaskets), it will leak and use more with a slippery synthetic inside.

Consider your type of driving. If most of your accumulated mileage is on the highway at higher, steadier speeds, a synthetic oil will reduce friction and probably help delivery better fuel mileage. Under such ideal highway conditions, the oil change interval can be extended, as long as the oil filter will operate effectively for the extended life of the oil. If the filter can't do its job for this extended period, dirt and sludge will build up in your engine's crankcase, sump, oil pump and lines, no matter what type of oil is used. If using synthetic oil in this manner, you should continue to change the oil filter at the recommended intervals.

Vehicles used under harder, stop-and-go, short hop circumstances should always be serviced more frequently and for these vehicles synthetic oil may not be a wise investment. Because of the necessary shorter change interval needed for this type of driving, you cannot take advantage of the long recommended change interval of most synthetic oils.

Finally, most synthetic oils are not compatible with conventional oils and cannot be added to them. This means you should always carry a couple of quarts of synthetic oil with you while on a long trip, as not all service stations carry this oil.

The latest development in synthetics, is a blend of conventional oil and synthetic oil. This provides the advantages of both oils at a cost significantly lower than a straight synthetic oil.



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

All 1975 and later models covered in this guide will perform happily on unleaded regular gasoline. Owners of turbocharged Buick vehicles may opt for unleaded premium fuel of at least 91 octane to protect against detonation. Since many factors such as altitude, terrain, air temperature, and humidity affect operating efficiency, knocking may result even though the recommended fuel grade 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.

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 diesels (runs with the ignition off) 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 specifications, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage, in addition to making the engine run hotter.

Diesel Fuel

Fuel makers produce two grades of diesel fuel, No. 1 and No. 2, for use in automotive diesel engines. Generally speaking, No. 2 fuel is recommended over No. 1 for driving in temperatures above 20°F ( -7°C). In fact, in many areas, No. 2 diesel is the only fuel available. By comparison, No. 2 diesel fuel is less volatile than No. 1 fuel, and gives better fuel economy. No. 2 fuel is also a better injection pump lubricant.

Two important characteristics of diesel fuel are its cetane number and its viscosity.

The cetane number of a diesel fuel refers to the ease with which a diesel fuel ignites. High cetane numbers mean that the fuel will ignite with relative ease so that it ignites well in an engine being cranked at low temperatures. Naturally, the lower the cetane number, the higher the temperature must be to ignite the fuel. Most commercial fuels have cetane numbers that range from 35 to 65. No. 1 diesel fuel generally has a higher cetane rating than No. 2 fuel.

Viscosity is the ability of a liquid, in this case diesel fuel, to flow. Using straight No. 2 diesel fuel below 20°F (-7°C) can cause problems, since this fuel tends to become cloudy at low temperatures, meaning wax crystals begin to form in the fuel. This is often called the cloud point for No. 2 fuel. In extreme cold weather, No. 2 fuel can stop flowing altogether. In either case, fuel flow is restricted, which can result in a no start condition or poor engine performance. Fuel manufacturers often winterize No. 2 diesel fuel by using various fuel additives and blends (No. 1 diesel fuel, kerosene, etc.) to lower its winter time viscosity. Generally speaking, though, No. 1 diesel fuel is more satisfactory in extremely cold weather.

No. 1 and No. 2 diesel fuels will mix and burn with no ill effects, although the engine manufacturer will undoubtedly recommend one or the other. Consult the owner's manual for information.

Depending on local climate, most fuel manufacturers make winterized No. 2 fuel available seasonally.

Many automobile manufacturers (Oldsmobile, for example) publish pamphlets giving the locations of diesel fuel stations nationwide. Contact the local dealer for information.

Do not substitute home heating oil for automotive diesel fuel. While basic characteristics of these oils are similar, the heating oil is not capable of meeting diesel cetane ratings. This means that using it might offer not only hard starting but engine knock; even under warm operating conditions. This could result in unnecessary engine wear or damage.

Further, furnace oil is not blended for operation at colder temperatures as most heating oil filters are located indoors. It could easily clog fuel filters with wax.

The equipment used in burning furnace oil does not contain the extremely fine machined surfaces or extremely tiny nozzle openings used in a diesel engine fuel system. Very small amounts of dirt and abrasives that will pass right through a heating oil fuel system could play havoc with your diesel's injection system. Finally, minimum standards regarding sulphur and ash that help keep deposits out of your diesel engine and minimize corrosion may not be met by furnace oil.

One more word on diesel fuels. Don't thin diesel fuel with gasoline. The result is the most highly explosive mixture possible in your fuel tank and unwarranted danger. Fuel thinned with gasoline may not adequately lubricate the injection system, leading to premature pump and nozzle failure and need for an expensive overhaul. Cetane rating will also be effected in an undesirable way.

It's best to buy No. 1 or blended No. 2 fuel for wintertime use. If you must use some means to keep No. 2 fuel from waxing, blend it with No. 1 or use a quality anti-waxing agent.