sounds to me like either the oil drain/feed to the turbo or your oil breather system.
Do you have an oil catch can? what size is the hole/breather on it?
I was running a 12mm oil breather on my car that was causing smoke on decel, got it enlarged to 1/2 inch and it fixed the problem. Id be trying that first as its a quick, cheap and easy fix. If that doesnt fix the problem then id be looking at the turbo (seals)