A used oil analysis (UOA) is the only way to know for sure. Blackstone Labs, costs $25. Although I have not tried Royal Purple, a lot of people here use it, of the oils I have sent in to be tested from all of my family's cars the lowest wear metals I got came with Pennzoil Platinum Synthetic.
My .02:
Best oil -- Pennzoil Platinum
Best oil for the money -- Motorcraft
That being said, in this Trac I run Mobil 1 0w-30 Advanced Fuel Economy because it needs all the help it can get. UOA's are fine.