There used to be a practice of walking the rotors of a 206 backwards for three revolutions immediately after shut-down although you don't seem to see it much these days. The intent was to turn the N2 turbine (turning backwards engages the freewheel and turns the engine) and rub any accumulated carbon build-up, particularly from the No. 6 and 7 bearing labyrinth seals. Try it and see if it makes any appreciable difference to your smoking problem. Don't bother with the gearbox vent orifice at this stage - it's too early. Nor is the problem likely to be the oil check valve, this has an entirely different purpose. FCU settings - umm, not likely. BUT, one thing you may consider is changing your oil, although I've no experience of AeroShell 560, I can assure you that Aeroshell 500 and 250 series engines make to very worst of bedfellows. I speak from experience, using Aeroshell 500 in a 250 Series engine just about guarantees excessive carbon build-up. My suggestion is go to Mobil Jet Oil II. Let us know how what happens.