See Figures 1 and 2
Jeep recommends the use of what are called "multigrade'' oils. These are specially formulated to change their viscosity with a change in temperature, unlike straight grade oils. The oils are designated by the use of two numbers, the first referring to the thickness of the oil, relative to straight mineral oils, at a low temperature such as 0°F (-18°C). The second number refers to the thickness, also relative to straight mineral oils, at high temperatures typical of highway driving (200°F 93°C). These numbers are preceded by the designation SAE, representing the Society of Automotive Engineers which sets the viscosity standards. For example, use of an SAE 10W-40 oil would give nearly ideal engine operation under almost all operating conditions. The oil would be as thin as a straight 10 weight oil at cold cranking temperatures, and as thick as a straight 40 weight oil at hot running conditions.
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 more than the cost 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 manufacturer'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 hinder this. Most manufacturers recommend that you wait at least 5,000 miles (8,000 km) 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. If your truck already leaks oil (due to worn parts and bad seals or gaskets), it will leak 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 deliver 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.
A prime requirement for gasoline is the use of unleaded fuel only. All the vehicles covered in this guide require the use of unleaded fuel exclusively, to protect the catalytic converter. Failure to follow this recommendation will result in failure of the catalyst and consequent failure to pass the emission test many states now require. The use of unleaded fuel also prolongs the life of spark plugs, the engine as a whole, and the exhaust system.
Fuels of the same octane rating have varying anti-knock qualities. Thus, if your engine knocks or pings, try switching brands of gasoline before trying a more expensive higher octane fuel. Fuel should be selected for the brand and octane which performs without pinging.
Your engine's fuel requirements can change with time, due to carbon buildup which changes the compression ratio. If switching brands or grades of gas doesn't work, check the ignition timing. If it is necessary to retard timing from specifications, don't change it more than about 4°. Retarded timing will reduce power output and fuel mileage, and will also increase engine temperature.
Basic engine octane requirements, to be used in your initial choice of fuel, are 87 octane, unleaded. This rating is an average of Research and Motor methods of determination: (R+M)/2. For increased vehicle performance and gas mileage, use a premium unleaded fuel, that is, one with a rating of at least 91 octane. More octane results in better performance and economy in these engines because the ignition system will compensate for its characteristics by advancing the timing.
Gasohol consisting of 10% ethanol and 90% gasoline may be used in your vehicle, but gasolines containing methanol (wood alcohol) are not approved. They can damage fuel system parts and cause operating problems.
The diesel engine in your Jeep is designed to run on No. 2 diesel fuel with a cetane rating of 40. For operation when the outdoor air temperature is consistently below freezing, the use of No.1 diesel fuel or the addition of a cold weather additive is recommended.
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 or that it ignites well 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, because this fuel tends to become cloudy, meaning wax crystals begin forming in the fuel. 20°F (-7°C) is often called the cloud point for No. 2 fuel. In extremely 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 wintertime 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 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 in some cases, home heating oil refinement levels equal those of diesel fuel, many times they are far below diesel engine requirements. The result of using dirty home heating oil will be a clogged fuel system, in which case the entire system may have to be dismantled and cleaned.
One more word on diesel fuels. Don't thin diesel fuel with gasoline in cold weather. The lighter gasoline, which is more explosive, will cause rough running at the very least, and may cause extensive damage to the fuel system if enough is used.