Forget the full synthetic, go with a syn blend. Best value going. Mobil Delvac Elite 10w30 is a great syn blend. I have seen some very stellar used oil sample reports on this stuff. Holds viscosity very well. All the engine OEM's are factory filling with 10w30 now. I am partial to Schaeffer 10w30 syn blend I use in my remanned Detroit 60, but the Mobil Delvac Elite 10w30 would be my second choice without reservation. Both have very good levels of moly in the add pack, which is a excellent, proven friction reducer, and good levels of zinc and phosphorus (excellent extreme pressure protectants), and good levels of detergency.
Used oil sample reading is as much an art form as a science. The key is tracking the trend lines over time. For my S60, I get good, steady data numbers till around 22,500. Wear numbers follow a study trend, TBN depletion follows a study trend, viscosity fluctuations stay study, etc. Above that, and the numbers deviate from the trend. The oil is still good, technically, but various numbers are starting to go outside the study trend. So I settle on 22,500 miles / 450 hrs as my OCI limit. I use bypass (FS2500) for keeping soot down, but bypass will not filter wear particulates nor slow TBN depletion or prevent viscosity changes due to shearing and such. Each engine, even within the same brand and model, will be different. One has to get good, solid sample data over time to make a determination what they can do.