I wouldn't use oxidation to tell the life of the oil, especially since I've found most labs don't properly calibrate their machines to read oxidation accurately with synthetic oil.
With the lab I use, the oxidation numbers are always 2-3 times higher than they should be, even Terry Dyson has noticed that when doing my interpretations.
This brings me to another point, if you truly want to find the right interval for your car, and push your oil changes further, you should be using Terry's services, he truly knows how to read UOAs better than anyone else out there.