I have no proof just but to my ears Castrol Syntec has made my engine quieter than PP, Royal Purple. I know Syntec doesnt get much love here but it works great for my 06 Accord V6 6 speed.
I'm using Castrol Syntec for my Accord and love it. Syntec is not highly regarded here but from my own experience it makes my car feel great. I would say Pennzoil Plantium is a very close 2nd.
I have used royal purple before and it close to the performanc of the syntec but at double the costs...