The Lowdown on Motorcycle Engine Oil

PART I: Viscosity and service grades or motorcycle oil

Which motorcycle oil should you use?
Which motorcycle oil should you use?Julia LaPalme

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 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, but that discussion will have to wait until the next issue.

“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.”

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 clutch 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.

Changing the oil on a motorcycle.
.Spenser Robert


Pressing the Reset Button on All Those Critical Additives
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.