The key point of difference can be summed up in two words “common sump,” (maybe I should have said “sumped up in two words?”) Motorcycles typically have one, while passenger cars do not. So what does this have to do with the fluid? Simple. A common sump indicates that the lubricant needed to protect vital components in a bike’s engine, gearbox and clutch are contained in one place. Since the fluid is stored in one place, it would stand to reason that there is only one fluid. On the other hand, a passenger car has multiple sumps and thus require multiple fluids (transmission fluid, gear oils, etc.) to protect the same vital components. So, critical components for motorcycles are lubricated with a single fluid, while passenger cars require multiple fluids to protect the same components.
Although motorcycle oils and passenger car motor oil are similar in their composition, the additive packages are balanced differently due to the type of performance and protection they must provide. Since motorcycle oils are lubricating three different components with a single fluid, they require a highly engineered additive package to address the unique requirements of each component. A typical passenger car oil only has to provide protection and performance for the engine of which fuel economy and emission system protection are the key priorities. As such, passenger car oils include friction modifying additives which reduce friction in specific areas of the engine such as the valve train and piston ring/cylinder. The reduction in friction yields fuel economy improvement. Also, passenger car oil contains lower levels of Phosphorus and Zinc. Both of these ingredients must be controlled to adequately protect the vehicles emission system.
Since a motorcycle is inherently more fuel efficient than a passenger car and fuel economy is not as important to the consumer, motorcycle oils do not require the use of friction modifiers. In fact, friction modifiers can affect wet clutch performance. “Wet clutch” simply means that the motorcycle’s clutch is immersed in oil. Should a clutch be immersed in an oil that includes friction modifiers (like a typical passenger car oil), these additives will be absorbed on the clutch plates and as a result, the clutch will start slipping. This will cause a loss of power transfer to the back wheel, overheating as well as increased wear. Motorcycle oils, which do not include friction modifiers, are specially formulated to provide the appropriate level of “grip” to the clutch plates, but still provide the protection needed to ensure proper functioning and years of reliable service to the consumer. In short, friction modifiers are good for passenger car engines but bad for motorcycles.
The other key difference is the level of antiwear additives used in motorcycle oil versus passenger car oil. Motorcycle oils typically require high levels of antiwear additives to provide a protective barrier, preventing metal to metal contact in the transmission gears due to the extremely high loads between the gear teeth. Direct contact of the gear teeth can lead to scuffing or scoring of the gear teeth surfaces while vibrations typical of high revving engines promote fatigue induced damage like pitting. Passenger car oils do not need to lubricate the transmission as there is a separate lubricant (automatic transmission fluid) charged with this task (back to the common sump concept). So, a passenger car oil will not have a high enough level of antiwear additives to lubricate and protect a motorcycle’s gearbox.
In summary, passenger car oils include additives (friction modifiers) that are harmful to motorcycle engine components and don’t include sufficient levels of antiwear additives that your bike needs to function properly. Formulating a motorcycle engine oil means finding the optimum balance between engine protection, clutch performance and gear protection due to a common sump. Conditions which are not considered when formulating passenger car engine oils. So, when choosing an engine lubricant for your bike, select one that is specially formulated for motorcycles. Your clutch and gearbox will thank you and the reward will be years of reliable service. Click here to learn more about Castrol Motorcycle Oil.