The following is an informative explanation to many of the terms we use in describing motor oil. I found this article in the Sept 04 issue of Turbo & High-Tech Performance Magazine. For the oil guru’s this is grade school stuff, but for many, myself included some of information provided a better grasp, understanding the basic vocabulary gives me even footing to communicate. Thanks for your patience. The acronyms and abbreviated terms on motor oil labels can quickly turn your mind to sludge. How do you determine what will produce the best performance in your car? All you need to understand is viscosity certification and performance. The first stop in selecting the best motor oil for your car is to determine the correct viscosity. The Society of Automotive Engineers (SAE) sets parameters for determining lubricant viscosities. Viscosity is defined as a liquid's resistance to flow at a given temperature. Since temperature affects the rate at which liquids flow, a viscosity measurement is meaningless without a corresponding temperature. There are two ways of measuring viscosity: Saybolt Universal Seconds (SUS) and Centistokes (cSt). (SUS) is the time in seconds required for 60 cubic centimeters of a fluid to flow through the orifice of the Standard Saybolt Universal Viscometer at 100F and 212F. A Centistoke is a unit of viscosity as measured in a capillary tube viscometer under constant pressure at 40C and 100C. A lubricant is measured at two temperatures to determine the SAE viscosity simulating ambient temperature flow and operating temperature flow. There are two basic types of lubricant viscosities: straight grade and multi-grade. Straight grade oils are labeled by a single viscosity such as SAE 30. Though viscosity will decrease as temperature increases, straight grade oils will have the same weight at ambient and operating temperature. Multi-grade oils are designated for increased flow at low temperature. They're labeled with two viscosities reflecting both their ambient and operating temperature separated by a "W". The weight of these oils will vary between the two temperature points, such as in the case of an SAE 5W30. Despite common perceptions that the "W" stands for the word weight, it actually stands for the word winter. An SAE 5W30 will flow like an SAE 5 at low temperature (winter). As their lubricant reaches operating temperature, it begins to flow like an SAE 30. The logic behind multi-grade oil is to provide excellent protection at both ends of the temperature spectrum. So how do you determine which viscosity to use? Engine tolerances are the primary factor in determining which viscosity is appropriate. Following manufacturer's recommendations for viscosity is the best practice an a stock engine that has not been modified. While there are many variables in choosing a viscosity grade for a modified engine, the following represents a general rule of thumb to find your starting point. Some have incorrectly believed that higher viscosity engine oil equates to increased protection and cooler operating temperatures. This is seldom the case. Using oil which is to viscous will increase fluid friction, which in turn increases operating temperature. Using too high a viscosity can also reduce oil flow and increase the oil's susceptibility to shear (losing an oils ability to lubricate). Both cause premature wear. Additionally it will rob horsepower and reduce fuel economy due to increased drag and produce an extra load on the oil pump. While multi-grade engine oils do provide significant benefits over straight grade engine oils, it's a good rule of thumb to avoid extremely wide viscosity spans. Use the narrowest viscosity span available to suit your needs. The wider the viscosity spans, the more potential there is for an oil to shear. Once you've determined the proper viscosity, you must decide whether you need "Certified" oil. The Automotive Petroleum Institute (API) and International Lubrication Standardization and Approved Committee (ILSAC) set performance standards for automotive lubricants. Most likely you've seen these acronyms on the back of a bottle of oil or in your owner's manual under lubricant specifications. A stamp of certification from these committees indicates the product you’re about to pour into your engine’s crankcase has been determined to meet or exceed a battery of standard laboratory test procedures. These committees work closely with the auto manufacturers to set the standards, consequently auto manufacturers require that cars under warranty use a lubricant that meets API, ILSAC, or possibly both committee’s specifications. The API licenses gasoline and diesel engine oil and gear oils. API gasoline engine oil classifications begin with “S” for spark ignition, while diesel classifications begin with “C” for compression ignition. Over the past four decades the API has defined to different service classifications beginning with SA for gasoline engines. Over time, as engines and regulatory requirements (i.e. laws requiring reduced emissions) have evolved; API has updated service classifications to address the changes. The API gasoline service classifications have historically been backward compatible-meaning a newer service classification will satisfy the performance requirements of older, obsolete classifications. The API service classifications are designed in alphabetical order, with the current service class listed as SL, which came into effect in 2002. A new API service class SM is currently on the drawing board and expected to supersede the SL classification in the not to distant future. There are different C-series classifications which vary depending on the type of diesel engine for which they are intended. Two-cycle diesel engine oil classifications will be followed by a “-2” as in CF-2, the current API classification for two-cycle diesels, four-cycle diesel engine oil classifications are logically labeled with a “-4” as with current CF-4 diesel service class. The API also classifies gear lubricants. There are currently four gear lubricant service classifications which vary by extreme pressure, (EP) additive levels and the severity of the duty for which they are intended. Most automotive applications specify either a GL-4 or GL-5 service classification. API GL-5 lubricants are most commonly specified for moderate to severe duty hypoid gears, such as those found in the differential of a rear-wheel-drive vehicle. GL-4 lubricants are typically specified for synchronized manual transmissions. It’s important to be careful not to use a GL-5 classified lubricant in an application which specifies a GL-4 lube. The reason for this has to due with the level of EP additives in each. GL-5 fluids will typically have a higher degree of EP additives which tend to be corrosive in high concentrations. The corrosive nature of these EP additives can attack soft metal such as bronze and copper found in applications which specify a GL-4 lubricant. However some manufacturers such as Royal Purple manufacture gear oils which can satisfy both GL-4 as well as GL-5 requirements – meaning they have the load carrying capability necessary for GL-5 applications, yet are non-corrosive to soft metals. Once you’ve determined the appropriate viscosity and your need (or lack thereof) for a certified oil, the next step is to look at lubricant performances. There are two primary components the impact a lubricant’s performance: base oil and the chemicals (additives) added to it. The ratio of base stock to additives ranges from 75/25 to 85/15, with base stock accounting for the greater volume. One way to elevate the performance of a lubricant is to use higher quality base oil. Over the past few decades synthetic base oils have become increasingly popular in the automotive market, in fact, synthetic oils are often the factory-filled fluid that comes with many high-end luxury and performance cars. Synthetic base oils provide significant benefits over mineral base oils due to their uniform molecular structure. The man-made molecules of the synthetic base oils provide superior oxidation resistance, better low temperature fluidity, and a lower coefficient of friction. While higher quality base oils do offer benefits, additive technology plays a much more substantial role in determining the performance of the finished product, for instance, mineral based oil with a superior additive technology will outperform synthetic oil with a mediocre additive technology. When researching any lubricant manufacturer’s Web sites, be sure to look for test data from external sources, such as universities and well known organizations. Independent outside testing is typically a better indicator of the “real world” results than a company’s internal test data. Your choice of lubricants will significantly impact the performance of your car. Be certain to know the correct viscosity and API designation (if needed). When selecting a lubricant for your engine, the best way to determine which grade of oil your engine will need is to refer to the vehicles service manual. However, with modified engines you would have to consult your engine builder to determine the clearances that were set on the bearings to determine the proper grade of oil. Also engines utilizing forced induction will also require a heavier grade. Selecting the proper lubricant for your engine will not only add performance, but also increase the longevity of the engine. Select a high performance synthetic oil if you care about the performance and protection of your engine. They’re more expensive but they’re worth it for cars you care about. For your $500 beater car, stick with the 99-cent stuff.