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It takes fuel to make an engine run, but it also takes oil to keep it alive. As we all try to maximize the service lives and fuel economy of our engines, it's important to understand engine oil's role in all this, whether you use conventional, synthetic or blended varieties. This is an amazingly complex subject, so for the uninitiated, we're going to share some basic information to get you started with our engine oil 101. We won't get into every last detail here, but we will arm you with the fundamentals.

Why It's Important for Your Engine

Engine oil is a real multitasker, fortified with additives that perform beyond the obvious mission of combating metal-on-metal friction and heat. It also fights "blow-by": The soot, exhaust gases and unburned fuel that inevitably enter the crankcase in small doses by sneaking past the piston rings. In any event, without engine oil, metal parts will quickly bend, bond and barbecue. Not good.

What the Numbers and Letters Mean

Engine oil used to be a lot simpler. The local auto parts store typically carried a few varieties of a few major brands, and that was about it. As engines have become more complex, engine oil has diversified to keep up with changing needs -- as evidenced from all the specs printed on bottles of engine oil. Fortunately, there are industry standards that explain it all.

The Numbers

On the front of a typical engine oil container, you'll see the oil's specified viscosity rating. For our basic introduction to engine oil, just think of viscosity unscientifically as "thickness." A few common viscosities include 0W-20, 5W-30, 10W-40, and 20W-50, though there are many more. These are multigrade oils, containing additives to tailor their viscosities to various engines' requirements and ambient operating temperatures (although, on the latter point, the numbers themselves do not translate directly to thermometer readings).

Take the example of 5W-30 oil. The first number, 5, indicates the lowest operational cold temperature range, while the second, 30, signifies the highest operational temperature range. You may see single-grade oils with simpler viscosity numbers, like 20W, for instance. These have a limited operating range and only perform well for specific applications and conditions. Most modern vehicles are happiest with multigrade oils.

The Letters

Whatever the engine oil's viscosity, the numbers are usually preceded by "SAE," which stands for the Society of Automotive Engineers, the organization whose viscosity standards determine the numbers. Incidentally, the prominent "W" is somewhat redundant in that it stands for "winter," reconfirming the first number's representation as the coldest operating range.

Some other letters you might notice on the container could be in the API "donut." API is the American Petroleum Institute, which established voluntary certification standards in a partnership with major American and Japanese manufacturers. The letter code on the donut represents the oil's performance level. For gas engine oil, you may see "API Service SM" as an example. A diesel-specific engine oil letter code might be "CI-4" or something similar. Generally speaking, oil with sequentially higher designation codes supersede previous codes and are compatible with engines built to run with oil from earlier designations.

That's not to suggest that everyone is OK with API. Other more rigorous and/or specialized engine oil standards have been put into play, and you might see their designations on the container as well. They include the European Automobile Manufacturers' Association (ACEA), the International Lubricant Standardization and Approval Committee (ILSAC), the Japanese Automotive Standards Organization (JASO), and even car manufacturers themselves.

Which Engine Oil is Right for Your Car

If you're beginning to suspect that selecting the right engine oil isn't straightforward, you're correct. For one, synthetic engine oils are now in the mainstream, though they've actually been available for decades. As the name suggests, they're derived from chemical compounds other than those present in crude oil. More expensive synthetics offer improved lubrication at all temperatures and are capable of extended service intervals. Recently, synthetic blends have become popular, combining conventional and synthetic engine oils as a mid-range compromise. Additionally, there's oil formulated for high-mileage engines, with additives to condition engine seals.

Even two identical vehicles may benefit from different engine oil viscosities and/or formulas. It partly depends on the car's engine (gas, diesel or hybrid), age and mileage. It also depends on how you drive, how much you drive and your climate. The easiest answer to this question? Consult your owner's manual. Some manufacturers outline very specific blends, brands and viscosities -- especially on their newer models. The designated oils may have specific additives or characteristics that your engine needs for optimal economy and performance. If you're ever in doubt, ask an authorized dealer or an ASE-certified technician who has experience with your car model.

How Often Your Engine Oil Should Be Checked and Changed

The slightly exaggerated answer to this query is that you can't check your oil often enough. Oil keeps your engine from going "boom" like an over-the-top Mythbusters experiment, so why wouldn't you make it a frequent habit to verify the level? For most cars, it's ideal to check the oil with the engine cold and not running (there are a few exceptions, so consult your owner's manual), while parked on a level surface. If you're not sure where your engine's dipstick resides, the manual will reveal this as well.

Pull out the dipstick and wipe it clean with a rag or towel. Replace it, then pull it out again, holding it horizontally. You should see oil between the two lines or holes in the dipstick; when it's in that range, your level is fine. If it's at or below the lower mark, add a quart and remember to check the level again soon. Again, a cold engine will usually give the most accurate reading, but if you incorporate this habit into your fill-up routine at the gas station, so be it.

Whether you change your own engine oil yourself (truly a vanishing art) or have it done for you, just be sure you do it. As the oil's additives break down and contaminants enter the engine, oil's effectiveness is seriously compromised. How often should it be changed? As with selecting the right engine oil, the frequency largely depends on what you drive and how you drive.

The once-standard schedule of every three months or 3,000 miles is, well, changing. Some manufacturers and engine-oil producers are recommending longer service intervals under certain circumstances. Of course, there are the traditionalists for whom even the old three-month/3,000-mile guideline seems painfully long. Suffice it to say, drivers are strongly divided on the issue. When in doubt, refer to your owner's manual. If you're still in doubt after that, talk to a qualified service technician. Finally, regardless of how often it's changed or by whom, be sure your used engine oil winds up at a designated collection facility.

Oil for Thought

If you think fuel is important, consider oil's role in your engine's well-being. There's far more to engine oil than we're able to cover here, but with this look at the fundamentals, you can begin to understand engine oil and make the right choices for your vehicle.

Tom Torbjornsen, a regular contributor to AOL Autos, recommends changing " ... regular petroleum based oils every 4-5 thousand miles and synthetic every 5-7 thousand miles." Tom has been in the auto repair industry since 1972. You can read more on what he has to say about engine oil change intervals here.



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