The beginners guide: Motorcycle oil

Engine oil is defined as "a liquid that reduces friction and wear between moving parts within an engine and also serves as a coolant." To that I'd add that it also functions as an internal housekeeper, collecting and holding debris until it can be deposited in the oil filter, and that it entrains and neutralizes acids and moisture created by combustion. Lastly, it offers protection against oxidation, especially when the engine sits for an extended period. That's a lot to ask of eons-old compost, but if it weren't for oil, internal combustion as we know it simply wouldn't exist.

Lubrication occurs when a thin film of fluid (in this case oil) is inserted between two solid surfaces that move in relation to each other. The film is usually no more than a few hundred microns thick (human hair is between 50 and 150 microns in diameter), and under great pressure. To some degree, all liquids can act as lubricants, including water, and in fact water can be pretty slippery, as anyone that's ridden a powerful bike on a wet road knows all too well. But that doesn't mean water would make a particularly good engine lube, in large part because it lacks viscosity.

Viscosity is defined as a liquid's ability to resist motion or flow. Water has very low viscosity; molasses, very high. A fluid with higher viscosity creates a thicker film between parts and bears more pressure than a fluid with a low one, so in general it's better able to withstand pressure, though too thick a film can induce power-robbing friction. Oil is thick enough to create a strong protective film, yet not so viscous that it creates a lot of friction between the parts, which makes it an ideal engine lubricant.

There is a catch though. Viscosity is affected by temperature, so different grades or weights of oil have different viscosities, and only work properly over a fairly narrow range of temperatures. For example, at 212 degrees Fahrenheit, straight 30 weight oil has a VI of 9.3 to 12.4, a 40 weight 12.5 to 16.2 and a 50 weight a VI of 16.3 to 21.8. As you can see there is some allowable variation so one refiner's 40 weight may be only slightly heavier than another's 30, or slightly lighter than a 50.

Because viscosity is affected by heat, oil that works just fine at its normal operating temperature (roughly 180°F) may be too thick to lubricate well when the engine is first started. This becomes a real issue when mono-grade oils are used.

For example, the normal operating temperature of a liquid-cooled engine is right around 212°F. At that temperature, a straight 30 weight oil has a VI of ten. Unfortunately, as the temperature drops to 104 degrees the VI increases to 100, making it that much thicker and resistant to flow. Worse, at 32 degrees—a temperature most of us see every winter—the VI is off the charts. In a practical sense it's frozen solid. During a cold winter start-up, straight 30 weight oil is so thick it can't flow properly into the tiny .0014 -.002 gap between a crankshaft journal and its bearing, until the engine has developed enough heat to thin it out.

Obviously, this isn't a particularly good situation, and it's estimated that up to 90% of an engine's wear takes place during cold starts, so anything you can do to enhance lubrication during that period pays huge dividends.

The initial solution was to use light oils through the winter, and heavy ones during the summer, but that was inconvenient and only partially successful. The eventual resolution was the creation of multi-viscosity oils: Oils that behave like light oils at low temperatures and heavier ones at high temperatures.

Multigrades, identifiable by designations that contain two numbers separated by a W signifying "winter" (for example, 10W30), are oils that have been infused with long chain molecules called VI improvers. These are usually olefins and polystyrenes (plastics) that curl into little balls at lower temperatures. At low temperatures the improvers have little effect on viscosity, but as the oil heats up they uncurl and interlace, preventing viscosity loss. The result is that 10W30 oil has less viscosity than a straight 30 when it's cold, though not so little as a straight 10 weight, so it's perfect at the engine's normal operating temperature.

The myth that multigrade oils are somehow less efficient than mono-grade oils is just that. Because they initially behave like low VI oils there's less viscous drag on the engine's internal parts, so starting effort is reduced, saving wear and tear on the battery and starter. Because they have better flow characteristics when cold, they provide better lubrication during warm-up, which reduces engine wear, and finally, because they behave like a high-VI oil when up to temperature, they lubricate just as well as the equivalent monograde. So bottom line is there's no reason not to use them in your motorcycle, no matter what that old geezer on the '53 Panhead tells you.

Another myth, or at least misconception, is that multi-grades somehow increase their viscosity as they warm up. They don't; they just lose less viscosity than a straight weight would at the same temperature. As you can see on the chart, a straight 10 weight starts with a VI of 30. It drops to a VI of 6 at 212°F, but since a VI of 10 is all it takes to protect the average engine, all we have to do is infuse 10 weight oil with enough improvers to prevent its VI from dropping below 10 when it's hot.

There is one small problem. Because the VI improvers are long, delicate chains of molecules, they're subject to lots of wear and tear. They get sheared by gears, for instance, and torn apart by the wiping action of cam followers. Consequently, as running time accumulates, the additives degrade and the oil "falls out of grade." In essence, your 10W30 turns into a 10W20 or worse, hence the need for regular oil changes.

Synthetic oils have been around since the 1930s but they've only come into their own in the last decade or so. Essentially there are two types of synthetic oils; Class IIIs which are based on petroleum oil stocks, and technically considered a mineral oil, and Class IVs, which are created in the laboratory. A third type, the semi-synthetic, is a blend of petroleum and synthetic stocks, and contains no more than 30 percent synthetic oil.

The principle advantage of a synthetic is that its molecular structure can be tailored to provide specific lubricating characteristics. Basically they're designer oils that can be chemically adjusted to a fine degree to deliver exactly the kind of lubrication their creators want them to. Call them Franken-oils if you want.

In broad strokes, synthetics generally outperform petroleum-based oils, especially when it comes to things like ability to withstand high heat, yet still pour well at room temperature, and cling to metal. They also resist shear well, which makes them, along with their near-relation the dedicated motorcycle oils (many of which are Class IV synthetics), attractive for use where the engine and transmission share a common oil supply. It also allows you to extend the change interval, subject to the engine manufacturer's approval. That being said, good petroleum oil, may, given the right make up, outperform a synthetic under certain circumstances, so there really is no across the board, "best oil."

Like anything misunderstood, there are plenty of myths about synthetic oils. One is that synthetics, "because they're slipperier" will create leaks. This one has been around since the 80s and in every case someone's previously oil-tight engine suddenly leaked like a sieve after they switched to a synthetic. Early on there were some seal compatibility issues with synthetics, but those have long been resolved. In my experience, synthetic oil is no more likely to create a leak in a sound, modern engine than conventional oil.

Another common myth says you should never break in a new engine with synthetics. When synthetics were first introduced this was true; the synthetic oils of the day just didn't allow the rings to seat, and many manufacturers recommended using conventional oils during the break-in period. Since a fair amount of new motorcycles now come with synthetics, I'd have to say that this one isn't as true as it used to be, but it still pays to break in a new engine using the (engine) manufacturer's recommended grade and type of oil. After that, pour whatever you like into it. In my studied opinion, that should be a synthetic, preferably a motorcycle-specific-type oil. I have to reiterate that synthetics, in my opinion, simply work better than most conventional oils, and particularly so in motorcycles that are ridden hard.

Let me qualify that by saying if you change your oil every 2000 miles, aren't especially hard on the iron, and your motorcycle doesn't specifically require synthetic oil, then a good petroleum-based oil is all you'll ever need. Under those specific circumstances, the additive package won't degrade too badly, there's no real need to use a synthetic. On that same note, my Honda VTX 1300 manual recommends using conventional 10W30 oil, and changing it at 8000 miles. That's a hard argument in its favor, and frankly while you can't go wrong using a synthetic, I'd probably opt for the conventional oil in that case, if only because I'd be changing it long before it ever saw 8k.

There's some controversy surrounding the use of motorcycle-specific oils, partly because currently only a few motorcycle manufacturers specifically recommend using them, and partly due to a test done in 1994. In that test case, the motorcycle-specific blends didn't perform any better than their conventional counterparts. A lot's changed in the last 19 years, and in my experience, the current generation of dedicated motorcycle oils are excellent. The major advantage is that their additive packages are blended to withstand shear better than automotive oils—an important factor when your engine oil is also being used to lubricate the transmission, which tends to hack up the VI improvers of conventional oils. Furthermore, because motorcycles are expected to sit for extended lengths of time with dirty oil in them, motorcycle-specific oils are often blended with extra corrosion inhibitors.

Anecdotally, I can tell you that in some specific instances I've noticed a genuine improvement in clutch and shifting action when conventional oil was replaced with something motorcycle-specific. Whether or not a dedicated oil offers any real advantages on a gently ridden, moderately powered bike that has its oil changed every 2-3000 miles is debatable, but a motorcycle specific oil will never cause harm, and is generally beneficial.

Oil Additives

Let's cut to the chase here: If oil additives were required, refiners would incorporate them, so there's absolutely no need to use them.

Metal Surfing

Now that we have a basic idea of what oil is, let's explore how it works. There are three fundamental types of lubrication. Hydrodynamic, or full film lubrication occurs when a wedge-shaped film of oil separates one surface from another. Think of the parts as surfing on a wave of oil. You'll find this type of lubrication between the crank journal and its rod bearing, and between the piston skirt and cylinder wall. Hydrodynamic lubrication is dependent on the oil's viscosity and the motion of the parts. If viscosity is too low, it'll be squeezed out from between the surfaces, and if the parts move too slowly, the oil will drain away. Hydrodynamic lubrication is a function of pressure, so unless the oil pump is making some, you've got nothing to prevent metal to metal contact except the residual film of oil clinging to the parts.

Boundary lubrication occurs when the oil pump has stopped turning or is turning slowly. For example, when an engine is started it takes a moment to build oil pressure, so during that period its only protection comes from the oil sticking to the parts. The oil's anti-wear additives, like Zinc Phosphate, form a very thin protective coating on the metal surfaces. That coating is just thick enough to protect things like main bearings, cam and tappet surfaces and piston skirts until there's enough oil pressure to create hydrodynamic lubrication.

Mixed lubrication is a combination of hydro and boundary lubrication. It occurs at very low engine speeds, for example at idle, when oil pressure and inertia drop off and the oil film can't fully support the load of the parts, so some of the parts, particularly the cam and tappets, have to depend on boundary lubrication to stay alive.

Oil reality

So what's the bottom line? First, although some oils are better suited to specific applications than others, there is no "best" oil for every circumstance. Second, you'll never go wrong following the manufacturer's oil recommendations. They have a vested interest in keeping your engine healthy and know exactly what oil their engines like. Third, no one ever damaged an engine by changing its oil too often. If you want to keep your engine healthy, regular oil changes using the recommended weight and quality of oil is the easiest way to do it.