The Lowdown on Motorcycle Engine Oil

PART II: Conventional vs. synthetic oils.

Conventional vs. synthetic motorcycle oils.
Conventional vs. synthetic motorcycle oils.©Rich Lee

Last time we discussed the fundamentals of engine oil and explored things like viscosity, the additive package, and service grades and standards. We brushed past a topic that's at the top of most motorcyclists' list when it comes to oil: the difference between conventional mineral oil and synthetic oil. 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 an 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.

“What’s the difference between mineral oil and synthetic oil, and more importantly, does it make a difference for your motorcycle?”

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 5 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.

What Oil is Best For Break-In?
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.”

Motorcycle oil analysis.
Motorcycle oil analysis.©Brian MacLean

Want to Know How Well Your Oil is Working? Send it to a Chemist.
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 for $28 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.