Have you ever tested that theory Duncan? Try warming a sample of each to 90 degrees, I think you will find there is a difference, especially the speed it will flow through a small hole under pressure.
Too thick an oil will obviously slow down oil flow to the cam chain, turbo, and anything else with a .9mm restrictor. Too thin will possibly flog bearings out and will drain past the drainback valve too easily causing cold start wear. If you can hear noise then there is definitely wear happening, which is why I stick to 5w40 winter or 10w40 summer. I have brand preferences also.
More importantly is to service your engine regularly. For the price of a semi synth these days ($20-$30) I would be dropping it every 4k and keeping it fresh, it's cheap insurance...