White smoke could be a stuck or sticky injector, perhaps it has some varnish on it or maybe there's some other crud caught in it or some water has oxidised part of it. I remember when this happened to my Jaguar, you couldn't see behind for the clouds of white smoke - bad fuel made the injectors gum up and they needed a sonic clean (twice, actually, and for 12 injectors it wasn't cheap).
The excessive oil might be caused by extra blow-by.
There's also the chance that there's a seal problem in the turbo bearings. Oil is pumped up from the engine to the turbo to lubricate the bearings. This should burn as blue smoke, you'd think ... how's the crankcase oil level? I'd watch that closely.
Might be an idea to fit a catch can and might be worth running an injector cleaner through now and again. Be interesting to see what the Nissan mechanics say the problem is.
Have you done an EGR mod, or chipped the engine?