Motorcycle engine oil reduced inside?

All engines burn some oil. It gets into the combustion chamber because piston rings and valve guides are not perfect seals. As long as it's not burning too much it's completely normal and nothing to worry about. That's why the owners manual tells you to check the oil level. If you didn't check in 2 years enough oil was burned to where it affected engine performance.

I am sorry but was exactly is "the oil level reduced"? Did you mean there was no oil in the engine? How much oil came out when you changed the oil? Who serviced the oil last time? OIl does not disappear. It either leaks or is burned in the engine or remains in the engine.

Additional: Read your owners manual too. Many motorcycles use back-in-the-day oils, API SF or SG because unlike cars, the oil is also used to lubricate the transmission. You can save a lot of money and headaches by just using regular standard SF or SG dyno oils.

Everybody who drives knows that engines use oil and worn out or defective engines use more. That's who nobody with the sense of a retarded monkey waits till they have problems to check the oil.

Since you decided to ignore the manual and use 0W40 in an engine that calls for 20W40 and didn't use mineral oil for break in like they told you to, you probably have glazed cylinder walls now and nothing but a hone will fix it.

its most likely burning oil, just because you dont see blue smoke or smell it doesnt mean it isnt burning oil. Check it regularly now. Bad valve guide seals can cause blow by and oil loss.