The Definitive Guide To Motorcycle Engine Oil

From the saddle, you’d never imagine the violence occurring inside your engine cases as your cruise down the road. There is a dizzying number of parts spinning and reciprocating inside your motor—some of them at hundreds of times per second. And all those components rely on a pitifully thin film of oil to keep them from turning to slag and bringing the whole mess to a screeching halt.

Oil is your engine’s lifeblood. Not only does it keep things spinning smoothly, but it also cools the transmission and pistons, helps the piston rings seal combustion pressure into the head, and even serves to neutralize nasty chemicals that are created after the air/fuel charge goes bang. There’s a lot more to engine oil than just slipperiness, so in this and the following installment of MC Garage, we’re going to dive deep into the subject.

The first thing most folks think about when they consider oil is its viscosity. Viscosity refers to the oil’s thickness—the higher the rating, say 50 weight, the heavier or more viscous the oil is. Viscosity is a critical factor in how well the oil flows and how much protection it offers. Modern multi-viscosity oils are kind of magical. They provide the right flow characteristics and lubrication across a wide range of temperatures, from frosty fall mornings to scorching-hot summer afternoons.

The “W” following the first number in 15W-50 stands for winter, not weight, and is a measure of the fluid’s flow rate at a seriously low temperature of -15 to -20 degrees Fahrenheit. So at sub-zero temps, 15W-50 will flow no slower than a 15-weight oil. This cold-weather behavior is critical to cranking speed and how readily the oil will flow during initial start-up in cold climates, though obviously it’s more relevant to automobile drivers than to motorcyclists. After all, who’s crazy enough to go for a ride when it’s below zero out? The second number in 15W-50 represents the oil’s high-temperature viscosity, as measured at a temperature of 212 degrees Fahrenheit. For 15W-50, that means the oil will be no thinner than a 50-weight oil at operating temperature.

More viscous oil provides a thicker film and better protects parts during normal operation. Your motorcycle’s manufacturer knows what viscosity range will meet the motor’s needs, so it’s important to abide by its recommendation.

How does a fluid defy the basic laws of physics and thicken when heated? Chemists mix in viscosity index improvers (VII) that expand and elongate when hot, thereby increasing the oil’s viscosity. So to create a 15W-50 oil a manufacturer would start with a 15-weight oil and stir in enough VII to make it thicken to 50-weight once hot. There’s both art and science in choosing the correct mix of modifiers, so the oil is equally competent when cold as when hot.

Viscosity index improvers are just one of many high-tech additives that get stirred into each bottle of oil. Besides the VII there are detergents and dispersants; detergents do a little light cleaning while the dispersants hold the junk in suspension so it cannot be redeposited in the engine. Plus there are buffers that neutralize acids, sacrificial lubricants that serve as a last-ditch barrier between metal-to-metal contact, solvents to break up impurities, and corrosion inhibitors.

These additives make up about 20 to 25 percent of the content in each bottle of oil. That’s right—only about 80 to 75 percent of each liter is actually oil, known in the industry as a base stock. If the base stock is refined from crude oil that’s pumped out of the ground then the finished product is categorized as mineral oil or conventional oil. If the base stock is synthesized in a lab, you’ve got synthetic oil. There are some pretty striking differences between mineral and synthetic oil that you can read more about in our comparison.

Take a look in your owner’s manual and you’ll find a recommendation for an API (American Petroleum Institute) service type and a JASO (Japanese Automotive Standards Organization) standard. These little letter codes might seem insignificant, but you don’t want to ignore them. The API classification refers to the automobile model years the oil was designed to work on. It speaks to things like lubrication properties, detergent properties, and other factors and gets updated every few years. All API classifications for gasoline engines start with an S, followed by the letter A through the current N standard. Buy an SA-service oil (not hard to find at gas stations and discount stores) and you’ll be running your engine on oil that the API warns “may cause unsatisfactory performance and equipment harm.” That admonishment extends to SD-classified oils as well. Most motorcycle manufacturers call for an SG rating (introduced in 1995) or higher, so always look for the latest designation when buying oil.

Traditionally motorcycles used oil designed for automobiles, but as fuel efficiency demands for cars increased, friction modifiers have been added to the oil package. Certain kinds of friction modifiers, however, are great for cars and light trucks but can cause the clutch to slip in motorcycles. Recognizing that certain oils were causing issues for motorcyclists, the JASO stepped up and introduced two standards for motorcycle oils based on the SAE’s (Society of Automotive Engineers) Clutch Friction Test: MA for bikes with wet clutches and MB for bikes with automatic transmissions. If your bike has a wet clutch you’ll want to make sure you see that motorcycle-specific MA classification. Conversely, you want to steer clear of MB oils and any oil that’s labeled as “energy conserving” since both blends will contain problematic friction modifiers.

It’s possible to find automotive oil with the appropriate API service type and viscosity range in a non-energy-conserving formulation, but that doesn’t necessarily mean it’s appropriate for use in your bike. There are some key differences between motorcycle engines and car engines, most notably the fact that motorcycles have shared sumps. The meat grinder that is the transmission is tough on the viscosity index modifiers and calls for high-pressure and anti-wear additives that aren’t part of the normal automotive-oil package. Add to that the fact that motorcycle engines make more power per liter, spin faster, and run hotter than car engines and it’s pretty clear that picking motorcycle-specific oil is important.

We all know that regular oil and filter changes are the best way to keep our engines happy, but why does oil need to be changed in the first place? “It gets dirty” is the simple answer (and true to some degree—soot from the combustion process blasts past the piston rings and clouds the oil), but there’s a bit more to it than that.

Remember those magical viscosity index improvers that allow a 15-weight oil to thicken to 50-weight once hot? Those fancy molecules are fairly fragile and tend to get chopped up in the transmission as the miles stack up, leading to viscosity breakdown and insufficient lubrication. Other additives get worn out and used up too. And while the base oil itself is extremely durable and will last for thousands and thousands of miles, it’s important to change the engine oil in order to press the reset button on all those critical additives.

Some people are as opinionated about oil as they are about professional sports or politics. But what's the difference between mineral oil and synthetic oil, and more importantly, does it make a difference for your motorcycle?

A mineral or synthetic designation refers to the base oil that makes up about 75 to 80 percent of each bottle of finished fluid. The other 20 to 25 percent of the bottle is additives like detergents, anti-foaming agents, buffers, viscosity index improvers, and sacrificial lubricants that provide a last-ditch barrier to prevent metal-to-metal contact. Nearly all additives are synthesized in a lab, but these additives aren’t considered when categorizing oil. It’s the base oil that makes the difference, and the American Petroleum Institute (API) has split them up into five groups based on key characteristics like sulfur content, saturate content, and viscosity index (VI). The VI isn’t the number that’s printed on the bottle but rather an arbitrary measure of the base oil’s viscosity change over a range of temperatures. The VI is closely related to the oil’s ability to reduce friction, independent of additives.

Group I, Group II, and Group III oils are all derived from crude oil that’s pumped out of the ground, while group IV and Group V oils are synthetic, meaning they’re concocted in a lab from chemically modified materials. The higher the group number, the less sulfur, the more saturates, and the higher the viscosity index.

Interestingly, some Group III oils can be classified as synthetics due to the fact that the API allows oils to be categorized based on performance rather than composition. Many oil manufacturers take advantage of this loophole and sell highly refined Group III oils as synthetics since it’s cheaper and easier to thoroughly process a mineral oil than it is to build a true synthetic.

So, the base oil determines if the oil is classified as mineral or synthetic, but what do those terms mean for your engine, and what are the pros and cons of each? Mineral oil has been around for decades, and it can offer great lubrication performance. As is the case with the gasoline market, the base oil that makes up most of the market’s engine oil comes from just a few major refineries. From there, each company adds its own additive package. By far the biggest appeal of mineral oil is its price. Since it takes less work to become a finished product, it’s pretty affordable stuff.

If you’re looking for the drawbacks of mineral oil, look to its origin. It’s refined from crude oil (which is a soup of everything from volatile liquids like gasoline to solids like paraffin), so mineral oil inevitably contains a variety of different molecules as well as some lingering impurities. The higher the API category the cleaner the base oil is, but in the long run conventional oils can’t compete with the purity of synthetics, and as a result they are less chemically stable and more easily oxidized and acidified. That means they can break down more quickly, especially under extreme use. And by “extreme” we mean high loads, high engine speeds, and high temperatures. Think motocross bike being ridden at full tilt on a scorching summer day.

Just as the cons of mineral oil revolve around its impurities, the benefits of synthetic oil revolve around its purity. Synthetic base oil consists of uniform molecules that are specifically tailored to serve as a performance lubricant. (Fun fact: Synthetic oils were developed as early as the 1870s but didn’t become commercially viable until the late 1940s. We have aircraft turbine engines to thank for the popularization of synthetic oil.)

There’s no lingering dinosaur dung or other leftovers—just pure lubricating goodness. That means synthetic oil does a better job of reducing wear, performs better under a wider range of temperatures, and is more resistant to thermal and chemical breakdown so has the potential to offer longer service intervals. The cons? There’s really just one: cost. Synthetics are difficult to produce, and the price reflects that. We’re talking $10 or $15 a liter versus about $5 a liter for mineral oil.

Torn between the two? Semi-synthetic oil is a mixture of mineral and synthetic oils. (Although as you’ll recall, some Group III oils can be categorized as synthetic, even though they’re derived from crude.) Semi-synthetic offers some of the benefits of synthetic oil but without all the cost. The thing is, there is no set minimum percentage for synthetic content, so semi-synthetic oils can range from as little as five-percent synthetic on up to a 30-percent blend. For the most part, though, semi-synthetic oil uses about 15-percent synthetic oil to help boost performance.

So the bottle will say what base oil it’s made from, right? Not likely. The only time you really know you’re getting the good stuff is when you see ester on the bottle. Ester isn’t your great-aunt; it’s a family of impressive Group-V molecules that have some important properties like high detergency (for engine cleanliness) and super-strong film strength that persists in the presence of punishing temperatures. And since esters are the best and most expensive synthetics to make, manufacturers are always going to tout it on the label.

When it comes down to it, there’s a lot of marketing that goes into oil, and it can be difficult to really know what you’re buying. Our advice? Buy oil from a reputable, established brand and change it regularly. After all, changing your oil on time is just as important as what oil you use.

Everyone knows that those first few hundred miles are a critical time for new engines, but besides the break-in procedure debate (by the book versus ride it like you stole it) there’s also some controversy over whether or not you can or should break in an engine using synthetic oil.

"From a technical standpoint it doesn't matter," says Bel-Ray's Andrew Hodges. "I have never come across any evidence that a synthetic cannot be used to properly break in an engine." Some say that synthetic oils are "too slippery" to allow the minute wear necessary to properly seat piston rings against cylinder walls. "That argument is nonsense," Hodges says.

“From a financial standpoint,” Hodges continues, “absolutely use mineral oil for break-in. Break-in barely even uses the oil, and you typically change it after only a few hundred miles. No need to pay for full synthetic on a short-change interval like that.”

If you don’t take much stock in advertising claims or anecdotal evidence on the forums, you’ll love the idea of oil analysis. This process entails sending a sample of used engine oil to a lab for testing. The result is a report card detailing your oil’s—and to some degree your engine’s— health. Blackstone Labs offers a standard oil analysis and will even send you a sample kit for free. The test establishes levels for wear metals, measures actual viscosity, and checks for contaminants such as gasoline or coolant, among other things. Oil analysis is common among trucking and heavy-equipment companies where fleet managers use the info to track internal engine wear and fine-tune service intervals to optimize profit.

Motorcyclists aren’t likely to get their oil analyzed on a regular basis, but even as an occasional test it provides valuable insight into how your oil of choice is holding up. If you poured 10W-40 into your bike but your 4,000-mile sample comes back as a 20 weight, you know you need to change your oil more frequently or switch to a higher-quality lube. For an additional $10, Blackstone will perform a TBN (total base number) test that determines how much of the oil’s original additive package is still around and working. Likewise, those results go a long way toward revealing how well the oil is performing in your engine.