The question or suggestion is often made that you can use any engine oil in a motorcycle. While it is true that you can use any engine oil in them, the real question is whether or not you want to or not. While no one denies that the engines of a motorcycle and an automobile are designed differently, many people deny that each application benefits from specially formulated products for each type of engine. Comparing motorcycle engines to automobile engines piece by piece gives a good picture of the differences and the lubrication challenges faced by each engine. Please keep in mind these observations are meant for general application and there are certainly exceptions to many of these observations.
- Motorcycle cooling systems can be liquid cooled, air cooled or a combination of the two. Some systems utilize a very simple liquid circulation loop to cool and pump through a radiator and others use a more complex liquid cooling system to more evenly cool the motor. Air cooled motors are the most simple and simple maximize surface area on the motor and rely on air-flow alone to cool the motor. In simple liquid cooled and especially in air-cooled motors, uneven cooling and excessive heat can apply extra stress on the motor and the oil.
- Modern automobiles exclusively utilize liquid cooling which are typically sophisticated to the point to evenly distribute and dissipate heat as efficiently as possible at all times. This means automobiles tend to stress the oil and the engine parts less than motorcycles due to better temperature control within the engine.
Engine Speed –
- Motorcycles often run at engine speeds of about double that of automobiles. Higher engine speed means higher potential for foaming and more stress on the engine and oil.
- Automobile engines tend to run at lower speeds producing less heat and less stress on the engine and oil.
Engine Size –
- Motorcycle displacement tends to be small and produce a relatively high amount of power per unit of volume; increasing the heat and stress on the engine and the oil.
- Even the some of the smallest automobiles are more than double the displacement of some of the largest motorcycle engines. They do typically produce higher horsepower, but relative to their size, the automobile’s power to displacement ratio is significantly lower.
Compression Ratio –
- Compression ratios of motorcycles are typically higher adding more stress than automobiles and resulting in more combustion blow-by into the crankcase. This increases contaminants and degrades the oil faster.
Shared Sump –
- Many motorcycles (though not all) have a shared oil reservoir for transmission and engine lubrication. This means the lubricant must be able to lubricate both mechanisms appropriately and therefore must have additional properties beyond what either engine oil or a transmission fluid would have individually.
- Automobiles have separate transmissions and are lubricated by different fluids. Engine oil is used in the engine and a transmission fluid is used in the transmission.
These properties are what I would consider the major differences between motorcycle and automobile design (that affect the lubrication requirements at least). There are certainly many other differences and the list could go on and on, but these are the main focus points that should be taken into consideration when thinking about what kind of lubricant to use in a particular machine.