piston rings more likely... what's the oil consumption been like?

if the turbo was failing all your plugs would be oily

head gasket's proveable by draining the coolant & having a look at it (as well as mayonnaise spotting)
 
I can't think of many ways of getting enough oil into just one cylinder if everything else is ok.....
 
whip the head off & have a look - that's the best way

it might be the cylinder had oil in it already, so a clean plug would just get coated