Changing your engine oil is probably the best protection that you can give your Jeep. Although most of us know this, engine oils are generally misunderstood and are often confusing to the average consumer. Hopefully, this article will help clear the air about engine oil design and help you make an informed decision about your next oil change. Engine Oil Function and Design Most people think that engine oil is just the slippery stuff that keeps your engine from wearing out. In actuality, a tremendous amount of technology goes into the design of modern engine oil so that it provides protection against numerous physical and chemical processes. Some of the primary functions of engine oil are: Prevent corrosion and rust Prevent varnish and sludge Neutralize acids Disperse contaminants Remove wear debris Remove heat Control oil consumption To provide this kind of protection, various additives (chemicals) are blended with oil (synthetic or conventional). Let’s briefly look at some of these additives: Anti-wear additive – Usually a zinc-phosphorus compound that provides a protective film on sliding metal contact points. Detergents – These are surface cleaners which may also neutralize acidic products of combustion. Dispersants – These chemicals help the oil absorb contaminants so that when you drain your oil, you remove contamination. In general, this causes the oil to turn black. Inhibitors – A chemical that will prevent engine oil breakdown due to heat and contaminants. This will prevent excessive oil thickening. Friction Modifiers – Some engine oils may contain friction modifiers to enhance fuel economy and provide low temperature wear protection at startup. Viscosity Modifiers – Most engine oils contain viscosity modifier to prevent oil thinning. As engine oil heats up, it becomes thinner, however, the viscosity modifier molecules expand with heat thereby decreasing the amount of thinning. These are the primary additives required to protect your engine. There are several other additives used to a lesser degree to improve engine oil performance: Pour Point Depressant – This additive prevents wax crystallization of conventional oils at very low temperatures, thereby improving engine cranking and engine oil flow. This additive is active at temperatures below 0 degrees F. Seal Swell Agent – This additive causes slight swell of rubber seals to prevent engine oil leaks. Anti-foamant – This additive prevents engine oil foaming by reducing surface tension. Most conventional oils will contain about 93% oil and 7% additives. Synthetic oils will tend to vary more and may have as much as 20% additives. Understanding Engine Oil Terminology OK, enough science. How does one know which oil is right for their vehicle? Most people do not really understand which viscosity oil they should use. Many people are brand loyal without really knowing why they like a specific brand. Most people don’t even know what their owner’s manual specifies on the subject of engine oil. Let’s discuss engine oil requirements: Viscosity Determination Let’s start with understanding viscosity. Your owner’s manual may tell you to use SAE 5W-30 engine oil. What does this mean? SAE stands for the Society of Automotive Engineers. They have created a standard for identifying the viscosity characteristics of engine oil. The 5W-30 part of the symbol is actually just a name. The W part of the name is an indicator of low temperature viscosity properties while the 30 part of the name is an indicator of the high temperature or operating viscosity properties. The numbers are just relative indicators to help the consumer. In other words, a 5W oil does not have a viscosity of 5, nor does a 30 weight oil have a viscosity of 30. What the numbers do mean is that the viscosity characteristics of a 5W oil are lower than that of a 10W oil. As well, a 30 weight oil has lower viscosity properties than a 40 weight oil. Its all very confusing, isn’t it? The best thing to do is use the appropriate viscosity for your engine and climate as recommended by the manufacturer. In general, newer vehicles will specify lower viscosity oils such as 5W-30 while older vehicles will specify higher viscosity oils such as 20W-50. This is because today’s engines are built with tighter bearing clearances to take advantage of the fuel economy benefits of lower viscosity oils. It is not really a good idea to use thicker oil in one of these engines because it will disrupt the oil flow characteristics of the engine and may create excessively high oil pressure. In an older engine that was designed with larger bearing clearances, it is appropriate and recommended to use a thicker oil to maintain proper oil pressure and provide adequate bearing film thickness. This is also true for engines that have a lot of miles on them and the oil pressure is starting to drop off. By using higher viscosity oil, you can boost the oil pressure back to normal levels and increase bearing film strength. Oil Quality The owner’s manual will specify an API service category for your oil, i.e. API SJ. This specification indicates whether an oil has been formulated for use in your vehicle’s engine. You can find the API service category on the API Service Symbol "Donut" with the viscosity specification.
The API service symbol is sequential such that an older vehicle may specify API SH while today’s oils are formulated for API SJ. In almost all cases, the newer API service category satisfies the performance criteria for a previous category. In other words, API SJ is good for API SH, SG, SF and so on. If your vehicle calls for SJ and you use SH, you may not get the protection you need for your engine because that oil was designed for earlier engines. There are usually two types of API service symbols. The S (Service) category as described above is for cars, vans and light trucks with gasoline engines. The C category is for Commercial applications in heavy-duty trucks and diesel engines. Examples of the C category are CF, CF-2, CG-4. If you have a diesel engine you should verify that you are using the correct API C category. Be careful with the C category since the latest category usually – but not always - includes the performance of an earlier category. Keep in mind that diesel engine oils are usually formulated differently than gasoline engine oils due to high soot contamination of diesel engines. These oils require additional dispersant and detergent to keep the engine clean. If your owner’s manual specifies the API Certification Mark, you should also make sure your oil satisfies this requirement. The API Certification Mark looks like this:
The Certification Mark means that special engine tests were conducted on the oil to verify its protection performance and that the tests were conducted in a statistically acceptable manner. If your oil does not have this mark, it may not meet the warranty requirements of your new vehicle. Your Oil Selection So you should now be able to go to the store and find an acceptable oil for your Jeep using your understanding of the SAE viscosity, API Service Symbol "Donut" and the API Certification Mark. The brand you choose is totally up to you as long as it satisfies the service criteria. If you want to do more research on your own, a good place to start is the API web site, www.api.org. Look under petroleum specifications. In the future, we can discuss the performance benefits of synthetic engine oils, aftermarket engine oil treatments and perhaps greases and gear oil. 
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