Most bikes have a wet clutch- Engine oil is used as a lubricant for the clutch too.
If you use a thinner oil than recommended, it can mess up the clutch smoothness, and make a rough ride. Loose chain plays a big role too!
1 role of engine oil is to cause friction to happen smoothly, without any internal parts touching each other and to make sure that gears are changed properly etc. it is deffo the engine oil:)
The better the oil is developed/formulated the more properties it will have, and will fulfill your needs more:P