First the Basics
Obviously, motor oil is a lubricant. But that can mean a lot of things, including reducing friction. So the first job of any engine oil is to keep moving parts as separated as possible. One of the interesting things about this is that as engine manufacuring technology has progressed during the time motor oil has been developed also, machined finishes at the microscopic level have improved. This means fewer mountains and valleys, making oil's job easier and viscosity then less critical. Motor oil is also burdened with other tasks however. For example, it must fight corrosion, resist oxidation, cool, seal, and clean. A lot could be said about these things, because they are a lot to expect from an oil. In fact, the wonder of modern engine oil is that it performs these many wonders.
An oil's viscosity has traditionally been viewed as its major friction-fighting property. Oddly enough, its viscosity thickens when the oil is cold and thins when it is hot, unfortunately exactly the opposite of what an engine needs the oil to do. So right off the bat, motor oil "has its job cut out for it." Multiviscosity motor oil addresses this fundamental problem by having chemicals added to it (long-chain molecules called polymers) which cause it to actually, when hot, thicken, so that it is thicker than it otherwise would be. That is, it thickens less when cold, and thins less when hot, than a straight weight oil. Such an oil has a high "viscosity index," or resistance to thinning. On a graph, the multi-vis is depicted by a straighter line, while all single-weight oils have steeper lines of change. The way I used to show this to students was by showing a graph of the viscosity change of water, which doesn't change between freezing and boiling. Its line is perfectly horizontal between those two points. Then compare that to 30 weight oil, for which the change is dramatic cold to hot, thus its line crosses almost at 90 degrees. Multi-vis is in-between these two extremes, closer to the horizontal line of water, say at 25 degrees off horizontal. Much less change, in other words, than any straight weight oil.
The dual designation in a multi-vis oil, besides being a tip-off that it is multi-vis, communicates these viscosity characteristics at long-established temperature extremes. With a 10W-40 for example, the oil will thicken no worse than would a 10 weight oil under cold conditions, and thin no worse than would a 40 weight under the hot conditions. The "W": by the way stands for winter (not weight. The winter designation is the result of a 0 degree F test procedure. The second number, in this case the 40, is the viscosity tested at over 200 degrees F.
Another one of motor oil's inherent drawbacks is that it is to varying degrees quite volatile, meaning that it has a tendency to boil away into a gas when churned and heated. The industry standard allows up to 15 percent, and powersports engines are mostly right at this limit. In fact, powersports vehicle oil consumption is mainly due to vaporization, and hardly at all due to burning in the combustion chamber, and the engines which vaporize their oil the most are those in single-cylinder, high-output off-road bikes. In order to do more than simply lubricate, today's motor oil must also contain anti-oxidants, anti-foam agents, corrosion inhibitors, chemicals that fight the interaction of the oil with oxygen that can form sludge, and other ingredients designed to "keep house" within the engine. Detergents scour up contaminants, and dispersants keep these contaminants in solution until they can be emptied out with the oil change. Without these many hard-working additives, modern motor oil would simply be useless. And engine oils have had them since the late 1940s. All this is just the beginning. Clearly, petroleum motor oilís' fossil material base stock is by itself inadequate to meet modern engine lubrication needs. Additives must be included to the base oil to make the final product perform as desired. Unfortunately, these chemicals don'ít actually improve the oil, they just temporarily change its behavior. They eventually wear out. When they do, deposits increase suddenly, wear ratchets up, and all the other expected things happen.
Synthetic Motor Oil
In the early days of non-conventional motor oils, the most exotic examples got all the limelight. Oils containing graphite and others claiming miracle engine parts "re-plating" abilities quickly came to represent, however inaccurately, the the then-new synthetic lubricant industry. Unfortunately, in the face of this "snake oil" spectacle, honest hard-working synthetic motor oil products were overlooked, and trust in really good synth motor oils was slow in coming. Today, powersports OEMs themselves sell their own synthetic motor oil, and the situation has ultimately leveled out. Automotive and powersports OEMs both ship many models with synthetic oil these days.
Petro motor oil production starts when crude oil is separated into several different products, including propane, diesel oil, lubricating base stocks and other things. Two kinds of lubricating base stocks are produced, distinguished primarily by their viscosity and volatility. One of these is purchased by a motor oil developer, who adds to it a commercially purchased package of additives such as were described earlier (i.e. anti-wear, anti-foam, corrosion inhibitor, etc.). These additives give the base stock attributes it did not have before, and make it more suitable as an engine lubricant. As already mentioned, the additives are short-lived, and they vary from one retailer to another. Synth motor oil on the other hand is made very differently. The petroleum scientist starts with the same base stock, but he kind of reverse-engineers it. He breaks it down chemically into its molecular parts. The petroleum is divided, and divided again, until the remaining part is the smallest piece that still possesses lubricating quality. This piece is an ester, a sort of greasy group of atoms. Then the scientist rebuilds the base stock with high-end petroleum and chemical compounds. At this point it's still just a base stock, but the result is a base stock that already, without additives, is superior to the additive-laced finished petro oil product. This base stock is then sold to the motor oil developer who adds to it and tweaks it until it is a complete synthetic motor oil suitable for his label. Because the base stock is so good however, very few additives are needed. Two things result from all this. First, because the base stock was synthetically built, basic lubricating properties such as viscosity index and volatility index, in particular, are already superior, making this oil better at handling heat and at resisting vaporization, to name just two qualities. Second, the better base stock eliminates the need for the huge load of fancy additives that petro oil requires, meaning that there is less "junk" swimming around in the oil to take up space and wear out quickly, resulting in an oil that is more, well, oil, and one that lasts longer. In fact, higher heat carrying ability, less volatility, and fewer required change intervals remain the synth oil's major attributes even today. Incidentally, a good synth oil also has more pourability, meaning that it pours easier at cold temperatures, which is why it is used in equipment on duty in the Antarctic.
There is one outcome of using synthetic oil that not many talk about, and that is initial high oil consumption. Even oil engineers admit this happens. When a synth oil is introduced to an engine that has until then been using mineral oil, an interesting thing often happens. Sludge and varnish that has built up around engine parts is washed away, temporarily changing engine clearances, and also temporarily exposing the new oil to consumption by the combustion chamber and through blowby. This is only a temporary occurance however. By the second or third oil change things will normalize.
Special Powersports Issues
Whether you use petroleum or synthetic motor oil isn't really the most important issue. The fact is, there is one important difference in powersports-specific motor oil you need to know about. Unlike cars, most four-stroke powersports vehicle engines combine the engine and transmission together in one housing. Consequently, these engines use one lubricant to lubricate parts that in other kinds of vehicles, most notably cars, Harley-Davidsons, and some older Euro bikes, are lubricated by two or three different oils. Unfortunately, the automotive industry drives the engine lubricating industry. That is, it calls the shots; it determines the rules. The problem is, in recent years, the Environmental Protection Agency (EPA) has forced the engine lubricating industry to make motor oils that are so slippery they actually can improve the fuel economy in car engines, which is in an emissions issue. However, the motor oil companies are using friction-reducing chemicals in these modern auto lubricants to make this happen. These friction modifiers are highly incompatible with the oil-bathed clutches and sprags (one-way bearings) that are used in most powersports engines.
To address the unitized clutch issue primarily, the powersports industry in 1998, after recognizing that the auto lubricant industry is no longer supporting the powersports market, established special powersports-specific motor oil standards. All powersports vehicle manufacturers today categorically bar the use of today'ís automotive spec motor oil, from API (American Petroleum Institute) classification SJ and forward, in their products. This new standard is known as JASO, and it is basically an exemption from the govt. mandate toward energy-conserving motor oils. The JASO motor oil rating of most interest to us is JASO MA, though there is an MB classification also, as well as others targeting two-stroke engines.
The bottom line? No matter what oil you use in your motorcycle, ATV, scooter, or what have you, it needs to be an oil with an API service rating of SH or earlier, because that was the last non-energy conserving classification. However, such an oil is hard to find, though certain traditional powersports oil companies (Torco, Spectro) are still selling it. Far easier for most of us, since most of today's oils have the later API service rating such as the current SN classification (every few years the classification progresses in letter designations), is to deal with the API's ruining things for us by looking for the third party JASO MA stamp somewhere on the container. This stamp overrides the API classification. It communicates that though the oil was developed to the desirable latest API standards commensurate with increasingly more rigorous engine load requirements, the oil did not have added to it the federally-mandated friction-reducing components that are a universal part of automotive oils today.
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Motor oil, a mechanic's view