This is not my opinion, just taken from an oil manufacturer: Ask Mobil
Difference Between Car and Motorcycle Oils
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Question:
Difference Between Car and Motorcycle Oils
What is the difference from motorcycle oil than car oil?
-- Matt Coffman, Attica, NY
Answer:
Motorcycle oils and passenger car oils are very similar, with the exception of a couple of areas that are key to motorcycle operation. The first area concerns common sumps, or the use of motor oil, to lubricate and cool the transmission. As you know, in a passenger car the transmission is lubricated by an ATF fluid, which has frictional properties required for transmission operation. In a motorcycle, where the transmission may be lubricated by the engine oil, an engine oil that does not have the same level of friction modification (for fuel economy) of a typical passenger car engine oil will provide better transmission performance in terms of transmission lock-up and slippage. So motorcycle engine oil does not contain the friction modifiers of a passenger car engine oil. The second area of concern for motorcycle engine oils is that they tend to shear (breakdown viscosity) more quickly than a typical passenger car. Mobil 1 motorcycle oils are designed to provide exceptional protection against viscosity loss.
My opinion: Seemed both type of oil narrows down on viscosity issues.....so if you will use a full synthetic 4 stoke oil, viscosity concern will be eliminated...isn't it?
