® Motor oil in 2025


Manufacturing technology has progressed to the point that machined finishes at the microscopic level have vastly improved, theoretically making motor oil viscosity less critical than before. But it's still important. It's still the major consideration in engine oil formulation, not to mention labeling. Even in this day of more advanced engines and very high performing multiviscosity and synthetic oils, viscosity is an engine oil's fundamental attribute. And it is affected more by the oil's crude petro origin--its base stock--than by the additives put into it during formulation. Unfortunately, consumers have little access to that kind of information.

An engine oil must do more than simply lubricate. Today's oil also contains anti-foam agents, corrosion inhibitors and antioxidants. This last is a chemical that fights the interaction of the oil with oxygen that can form sludge. All of these additives "keep house" within the engine. There are even detergents that scour up contaminants, and dispersants that keep these contaminants in solution until they can be emptied out with the oil change. Engine oils have had these additives since the late 1940s, and their concentrations have increased every three years since then, as prompted by new vehicle warranty provisions and identified by the changing American Petroleum Institute service ("S") designations.

Probably the most amazing additive is that which manipulates the oil's viscosity. Motor oils with this additive are called "multiviscosity" oil. Multiviscosity oil is not two oils. It's a dual-stage oil, a single viscosity oil that is engineered to to have different viscosities at different times. A 10W-40 motor oil for example behaves in different temperatures as two different viscosities. At 32 degree F, it's a 10-weight oil and at 212 degrees it behaves through added chemicals like a 30 weight oil. The chemicals are polymers that expand when heated, bulking up the oil to overcome its natural thinning and thus increase its load-bearing ability.

Oil isn't like this naturally. It is inherently thick when cold and thin when hot--just the opposite of what the engine needs. Multiviscosity oil overcomes this. On an X, Y right-angle graph, water, which of course doesn't change viscosity at all between freezing and boiling, would be represented by a perfectly horizontal line between 32 and 212. A singly-rated oil's viscosity change by contrast is the most dramatic cold to hot, not a perfectly vertical line but much closer to one. Multi-vis oil is in-between these two extremes, not the same as but much closer to the horizontal line of water. Much less voscosity change, in other words, than any singly-rated oil. This resistance to viscosity decrease due to temperature the engineers call "viscosity index". Multiviscosityoil has high viscosity index.

Although engine oil is marketed in terms speaking of its behavior in different temperatures (the "W" stands for winter), engineers talk among themselves about flowability and load bearing. These two attributes are at odds with each other. A low viscosity will flow best and cool best but can cause more metal-to-metal contact and higher oil consumption. A high viscosity will reduce wear best but result in slower engine warmup, increased fuel consumption and higher maximum engine temperatures. A compromise must therefore be struck that puts as many positives on the table and the fewest negatives. In a given engine. And this is important. This happy medium won't look the same in every engine design. Not all engines flow their oil equally well and not all need the same emphasis on wear reduction. This is why there are so many engine oil choices out there.

Speaking of choice, one can choose either conventional or synthetic oil today. Petroleum engine oil production starts when the refinery's base stock is purchased by a motor oil developer, who adds to it a commercially available package of additives. These additives typically make up a sizable part of the oil's volume and give the base oil attributes it did not have before. Synthetic engine oil however is made differently. There the petroleum scientist starts with the same base stock, but 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, an ester, a sort of greasy group of atoms. Then the scientist rebuilds this 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 oil that already, without additives, is superior to the additive-laced petro oil product. This synthetic base stock is then sold to the motor oil developer, and very few additives are introduced. Two things result from all this. First, because the base stock was synthetically built, the oil's viscosity index is already superior, making this oil better at resisting thinning with heat.

However, viscosity and petroleum versus synthetic are minor concerns compared with two more issues. First is the JASO consideration. Unlike cars, most four-stroke powersports vehicle engines combine the engine and transmission together in one housing. 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, almost thirty years ago, the Environmental Protection Agency (EPA) forced the engine lubricating industry to start making engine oils that are so slippery they actually improve the fuel economy of car engines. The oil developers are using friction-reducing chemicals to make this happen. Friction modifiers are highly incompatible with the oil-bathed clutches and sprags (one-way bearings) that are used in Japanese powersports engines. To counter this, the Japanese segment of the powersports industry in 1998, after recognizing that the lubricant industry had chosen a course harmful to the powersports market, established special powersports-specific engine oil standards. All the Japanese motorcycle manufacturers now discourage the use of automotive spec engine oil, from API service classification SJ and forward, in their products. This unique powersports standard is known as JASO, and is an alternative (even an exemption) to the EPA mandate of energy-conserving motor oils. The JASO rating of interest to us is JASO MA. Theoretically, you should use only "MA" stamped oil in your Japanese bike's engine. However, today, twenty-seven years after the Japanese segment's override, powersports engine oil developers are widely meeting the MA standard yet many of them are not affixing the appropriate label. The MA designation was designed to make things clearer, and for a while they were. But now they aren't so much. Be diligent. If your chosen oil doesn't make the clutch slip, it is almost assuredly MA spec.

However, the second thing, and even more important, is maintaining your oil. Do you check the level often? Do you run the engine before determining the final level during an oil change? Some vehicles demand this more than others, but all of them have sumps, oil filters, and other engine system characteristics that require careful double-checks during the oil change procedure. And on some models, you run the engine then shut it off and immediately check the level, while with others, you wait a prescribed amount of time after turning the engine off before actually checking the level. In some engines you simply insert the dipstick in the hole. In a few others it is screwed into the hole to check the level. There are variables. And of course it must be done the way the manufacturer prescribes. Engine oil is also fairly volatile, meaning it has a tendency to boil away into a gas when churned and heated. This makes oil level monitoring even more critical. Oil consumption in motorcycles is mainly due to vaporization, and hardly at all due to burning in the combustion chamber. And over-filling has harmful effects as well. Crankcase pressure dan increase, leading to seal and gasket failures and contamination of the air intake system. Over-filling also impairs shifting in Japanese engines because it alters the clutch action.

All of these things are very important: the superiority of multivis, being aware of volitility, synth oil's advantages, the importance of oil level, and the MA stamp. But, if you run your bike out of oil--well, you can forget all of that. Nothing is going to help you. My experience is there are always folks who will let their oil level get too low. It's way too common. Check the level often. If you have an oil cooler, best practice is to check the level before the cooler has had time to drain down into the crankcase and give a falsely high reading. Because, the simple fact is, the single most important thing to remember about your engine oil is this: keep enough in there. Ask any career mechanic. In the repair shop, it is many times more common to see issues connected with the lack of oil than it is with all the other oil variables combined.


Last updated October 2024
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