When was the last time the rings were replaced?
These engines need ring and piston replacement sooner than you may think. The pistons have a very short skirt and wear very quickly. When they wear, they start to rock in the cylinder and wear the sharp edges of the rings down. If I were you, I would check the top end for wear. The recommended oil viscosity for your engine is 10W-30. 20W-50 is a bit heavy unless you are riding in very hot conditions.
Also, check the valve guides for wear and replace the valve guide seals.
If you wait too long to disassemble your engine, it will disassemble itself. :yikes: