From reading here and what I've noticed is it's all application specific.
My Matrix, I'd like to go 7,500-10,000 mile OCI's on a good synthetic, however it calls for a 5w20 or 0w20, and I'm not sold on a good synthetic being robust enough to protect in Florida heat at an Xw20 weight. I am awaiting an analysis of Castrol Edge HM 5w20 with 6,200 miles to see how that oil looks. Those results will affect my future judgement. Current OCI is Mobil1 EP 5w20.
My G20, I am fully confident I can do 5,000 with Pennzoil Platinum Euro 0w40. Probably could go 7,500. But for now I am okay with changing it out ahead of time because the cost isn't very high and still exceeds my original thinking of 3,000 mile OCI or the engine explodes
The Jeep however, gets changed every six months or 3,000 miles due to the mileage; 282K and going. I will probably stick with Valvoline Maxlife 10w40 though. The HDEO 15w40 seems a bit thick and I diluted it with some Pennzoil 0w20 I had on hand.
SO! All my reading and (limited) experience leads me to think 3-10K OCI's vary wildly.
Big reason as to why there is no "Best oil"