If it's dark gray to black it's fuel. Most Zs (with chip tunes at least) tend to smoke at higher boost because the tunes are intentionally rich for safety. This is only going to be more obvious when you rip out your cats and precats (as is the case with 700s).
If it IS oil, I'd probably lean towards turbos... a ring problem should show up in a leakdown test (though I could be wrong). Do you have the correct oil lines w/ restrictors installed?