We put outboard motor oil in our engines to serve several purposes. First, obviously, oil acts as a lubricant. If your engine is operating correctly, there is almost no metal to metal contact – everything is riding on a thin film of oil. However, oil has several other important jobs to do. Oil circulates throughout your engine, and cools parts that cannot get near a water jacket. For example, it’s becoming common in sport bikes to spray oil on the underside of the piston to cool it. There are no water jackets at all in your transmission. Motorcycle transmissions are oil cooled.
Your piston rings do not do a perfect job of sealing. Some combustion by products will slip past the rings into the engine. This can be little particles of carbon. Remember, diamond is carbon that was combined under heat and pressure. These little carbon particles can be quite damaging to your engine. Another job of your oil is to hold these particles in suspension until the oil filter can grab them. Also, if your gasoline has sulpher in it (it does), this sulpher can react with water and oxygen to make sulphuric acid. This is some stuff that is seriously bad for your engine. Your oil has special ingredients in it called buffers to neutralize acids. Finally, your engine can get internal build ups of tars, waxes, and other gunk. Your oil has solvents to try to dissolve this stuff and get and keep your engine clean.