Send a sample off to these guys:
Blackstone Labs in the free bottle they send you, and they'll figure out how long your oil is lasting, acidity, metals, dirt, viscosity, flashpoint, fuel, antifreeze, etc. On their advice I've stretched my oil changes from 4,000 miles out to 7,500 miles on the allroad, which takes over 7 quarts of synthetic per change. The savings in oil money more than paid for the analysis, and I can tell myself I'm not hurting the (very expensive) turbos.