So, certain additives like Boron will influence noise level. I am not saying conventional vs. synthetic. First of all, there is no conventional oil that can pass these approval specifications. There was a thread in general oil section about Challenger SRT where Blackstone was saying that oil was OK, yet at 127K, engine was full of varnish. So I am not sure conventional will perform same as synthetic when it comes to oxidation, deposits etc.
As for Castrol, yes, Castrol 0W40 is very smooth oil. I used it for years. I have now in the Tiguan 0W30 as I managed to get it on sale last year, and that oil replaced 0W40 that was in the sump. However, from oil performance level, wear etc. I do not think you will find any difference between Castrol 0W30/40, Mobil1 0W40, Pennzoil 5W40 etc. Those are all top tier oils. You wills ee though difference in shear etc. I know one thing, while M1 0W40 was always noisier, it literally did not shear at all in 5K OCI in my VW's, and those are DI engines (VW CC I had, and this Tiguan).Also one UOA I did in my Toyota, M1 5W30 EP still kept KV100 above 11 in 10K.
I will replace in my BMW in 10 days or so PP 5W40 and put M1 0W40 just to see how it does. I will not do UOA of PP as I mixed that with a bit of BMW TPT 5W30 that I had from previous OCI, but will do UOA of M1 and Castrol 0W30 that is in Tiguan.