was it a slow decrease or did it take sudden dramatic drops?
I'd be looking for a few things... first of all, what grade oil are you using (API service grade) ?
second, have it scanned for codes, pay attention to MAP/BARO codes, if you have it scanned with an actual DIAG tool, rather than a code reader, look at the data from the sensors...
3rd, look under the hood and tell us if you see an oily slime at the boots on the turbo plumbing...
finally, can't count out driver habits