It sounds like it's sucking oil down the inlet valve guide on the overrun.
Check the inlet valve oil seal .
I'm not familiar with that bike ,it may not have a seal ,in which case it could be a worn valve guide .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."