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Are you sure it's not coming from chain oil dripping onto the exhaust ?
Your description seems to suggest a lot of crankcase pressure. I would start by looking in the air filter box to see how much oil is in there, a little can be normal but a lot could mean high crankcase pressure. The high mileage of your bike may mean the piston rings / cylinders are worn a bit which can cause high crankcase pressure.
If there was a problem with piston rings or valves causing excessive crankcase pressure this would be highest when the engine was working hard. On overrun with a closed throttle the crankcase pressure should drop and airflow through the filter box should be very low.I don't understand why closing the throttle but travelling at the same speed in the same gear should cause oil vapour to be expelled unless the vapour was being sucked in by the intake system when the throttle is open and then has nowhere to go when the throttle is closed. In this case the oil consumption would be abnormally high ??
So, does it use a lot of oil?