"API" stands for "American Petroleum Institute" and the letters you see in their ratings refer to different standards that the oil is required to meet. The lower the second letter, the lower/older the standard. So, from a technical viewpoint, "SJ" is better than "SG." The issue for bikes, is that the newest standard allow for more friction modifiers, like Moly, which make the oil slippery-er. Motorcycle clutches are bathed in engine oil, and often do not like that. There is also a "JASO" standard, which the Japanese government set. It's better for our purposes because it rates motorcycle oil in a separate category. Look for "MA" on the bottle for an M/C specific oil.
The easy thing to remember is simply buy whatever rating/weight that your owner's manual says.
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Petroleum-based lubricants continued to be used successfully in the earliest automobiles, and even then, extensive efforts were made to classify them. Viscosity was identified as one of the most important characteristics of a motor oil, and oils were divided into light, medium and heavy viscosity classes. As soon as instruments capable of measuring viscosity were introduced, the Society of Automotive Engineers (SAE) developed a complete viscosity classification system, which included 11 different viscosity grades: SAE 0W, SAE 5W, SAE 10W, SAE 15W, SAE 20W, SAE 25W, SAE 20, SAE 30, SAE 40, SAE 50, and SAE 60.
Oil viscosity grades with a "W" were formulated for cold temperatures, while viscosity grades without the "W" were suited for higher temperatures. Before the development of viscosity modifiers in the 1940s allowed the manufacture of multi-grade motor oils, motorists commonly had to switch viscosity grades in their vehicles with the seasons. U.S. oil companies have been marketing multi-viscosity oils, such as SAE 5W-30, SAE 10W-40 and SAE 20W-50, since 1945. These oils offer adequate protection in both high and low temperatures.
As the years went by, engine technology improved. In order to meet public demand for vehicles with greater fuel economy during the oil embargo of the 1970s, automobile manufacturers produced smaller, lighter cars powered by smaller and more efficient engines. When fuel injection became common on gasoline-fueled engines, it offered motorists quick starts and quick mobility. These changes placed even greater demand on the vehicle's engine oil as it was required to flow and reach critical components as quickly as possible.
Today, demand for high performance, fuel efficient and more envitonmentally-friendly vehicles is as high as ever. To meet this demand, vehicle manufacturers have produced more aerodynamic cars with reduced air flow through the engine. Although these vehicles are more fuel efficient, they exhibit higher engine operating temperatures than earlier automobiles, putting even greater stresses on the engine oil in the areas of oxidation stability, deposit prevention and wear protection.
As automotive technology continues changing, engine oil technology has had to change as well. Over the years, manufacturers of higher quality oils have begun switching from Group I base stocks to Group II and Group III base stocks. Like Group I base stocks, Group II and III base stocks are mineral-based, but they contain less saturates, sulfur and paraffins and have higher viscosity indices, meaning they perform better than Group I base stocks, particularly in the areas of thermal and oxidative stability and cold temperature service. The introduction of synthetic motor oils to the marketplace has brought superior quality Group IV and V base stocks to the blending process.
In order to address the changing service and lubrication requirements of modern automobiles, and to allow effective communication amoung engine manufacturers, the oil industry and consumers, the API Engine Service Classification System (ESCS) was established in 1970. The system was designed to classify oils according to their performance characteristics and type of service for which they were blended. Later, in 1993, the AI Engine Oil Licensing and Certification System (EOLCS) was launched, a voluntary program which allows marketers who meet the minimum performance requirements to mark their oil containers with the API certification marks.
API service categories for gasoline engines are comprised of two letters. The first letter is "S" for "service" and the second letter is assigned alphabetically according to order of development. Thus, the first service category of "SA" is the earliest, while the latest is "SL." Later service categories exceed the performance requirements of previous categories and can be used in place of the earlier ones.
Where Do Synthetics Fit In?
Synthetic motor oils offer the best overall protection for today's high revving, hard working engines, but how do synthetic lubricants fit into the big picture?
In 1877, the prominent chemist team of Charles Friedel and James Mason Crafts successfully used aluminum trichloride as a catalyst, creating the first known synthesized hydrocarbons. It wasn't until 1929 that Standard Oil Company of Indiana commercialized the process, but the endeavor was unsuccessful due to lack of demand.
The Zurich Aviation Congress became interested in the development of ester-bsed lubricants in 1937. The Germans, frustrated by the failure of petroleum lubricants during the cold weather of the Battle of Stalingrad, prepared and evaluated more than 3500 esters between 1938 and 1944. Meanwhile, in the United States, the first diester base stocks (a compound using two ester groupings) were in development at the Naval Research Laboratory.
By 1947, Great Britain had discovered the benefits of using diesters as lubricants in turboprop aircraft. Later, with the advent of highly sophisticated jet engines, research and development in the area of synthetic lubricants really took off, and various synthetic formulations were developed to meet the demands of the new engnes.
The clear benefits of synthetic-based lubricants in jet engines impressed lieutenant colonel and jet fighter squadron commander Al Amatuzio, and by the mid-1960s, he became interested in developing a synthetic motor oil for use in internal combustion engines. Given the significant differences between a jet engine and an internal combustion engine, it was a massive task, but Amatuzio was up to the challenge. By 1972, after several years of intense research and development, AMSOIL Synthetic Motor Oil was born, and it became the first 100 percent synthetic-based motor oil to pass American Petroleum Institute (API) service requirements.
When it first hit the market, AMSOIL was far ahead of its time, and Amatuzio found it difficult to market such a revolutionary product. But with the complexity of engines increasing, forcing smaller engine designs and ever-increasing engine operating temperatures, engines demanded a superior lubricant, and people slowly discovered it in AMSOIL. AMSOIL had found its niche and began to grow, later branching out into the syntheitc diesel oil, two-cycle oil and transmission fluid markets. Competitors were forced to take notice, and soon industry giants Mobil, Quaker State, Castrol, Valvoline and Pennzoil were marketing their own synthetic variations.
Today, synthetic lubricants continue to become more and more mainstreamn. In fact, to ensure its vehicles receive the best protection, Corvette requires the use of synthetic motor oil in its engines. Other high performance vehicle manufacturers have followed suit, and even General Motors, Ford and Chrysler require synthetic lubricants for certain vehicle components. The vehicles of the future will continue to have smaller engines, lower emission requirements, higher performance characteristecs and better fuel economy, and they will require lubrication quality only a synthetic can offer.
AMSOIL continues to lead the way in the synthetic market. Not content merely being "The First in Synthetics®," AMSOIL is dedicated to being the very best in synthetics as well. AMSOIL Motor Oils outperform competing conventional and synthetic oils, offering motorists the ultimate in wear protection, all-temperature performance, maximum fuel efficiency and extended drain intervals.
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Some VFR guys says this:
That's a lot to read, but I hope that helps!