if I remember right your tuned up pretty high for an 8v power wise ?
cant remember if your still on a standard turbo ? the seals could be a bit worn as well as having the recirculated crankcase oil vapour fed back in before the turbo inlet, so youll be seeing some of that also.
you seem pretty handy so if your worried about turbo you could drop the turbo inlet rubber pipe and also the exhaust downpipe to access both sides of the shaft you could then push back and forth to feel for excess play in the thrust bearing (should be very little play this direction maybe a few tenths of a mm(0.1-0.2mm)) then wiggle opposite ends up and down to feel for radial play(this will have more play, a new turbo may have 0.3mm so id expect 0.5mm on an old turbo, if it feels really sloppy might be worth a trip for a second opinion.
here are some vids for visual comparison(pretty good condition but my removed turbo at 30,000 miles but high boost use)
might be worth fitting an oil catch tank on the breather line before turbo, that might cut down some especially if your running high boost/power.
have a read in here
http://www.alfaowner.com/Forum/alfa...-line-mini-oil-catch-tank-rutland-motors.html
its a nice small unit should be easily placed somewhere on the stilo, and if you check it regularly the small size shouldn't be an issue.
if you go that way make sure you get something similar to the sintered bronze filter near the end not the foam filters .
well done for finding the leaking gasket , ive heard they can resonate like a reed valve and make this noise.
im not sure if they use any gasket sealant at all ?
seen people mention hermitite red on gaskets (light smear ) but not specifically on jtd inlet manifolds.
just had mine replaced on the jtd16v and there was no sign of sealant on the old manifold face?
a phone call to a local specialist would best confirm.
heres a pic of mine as it came off
no pics further away sorry and cleaned now anyway