During the combustion process, this sulfur can combine with oxygen to form sulfur dioxide, or sulfuric acid. This, along with other corrosive nasties, works its way past the piston rings and into the motor oil where it is pumped throughout the engine. To prevent these acids from destroying the internal machined surfaces, oil companies mix in anti-corrosion additives. These additives neutralize the acids being pushed into the oil and keep our engine innards nice and shiny. The American Petroleum Institute is an industry organization that, amongst other things, issues standards for petroleum products including motor oils.
Oil companies agree to label their products with the appropriate API standard.
This allows your motorcycle manufacturer to tell you that you need, for example, SH oil. API has been grading oils for years. The oil's viscosity is listed in the center of the donut. The API grade is listed on the upper half. Despite the best effort of your piston rings and valve seals, some of your oil gets into the air flowing through your engine.