IMHO : Tighter clearances = lower visco numbers , looser tolerances = higher.
I run 15w-50 Chevron Delo 400 for break-in and i am a huge Mobil1 fan.
Synthetic oils are neato-rific if you are using them with modern gaskets and modern seals (uber-plasistized-silicone derivatives ) if you --de-dino
oil, your "Cork-And-Rubber" gaskets may be subject to leakage.
Remember, -- With natural oils, --dead-dinos leach into cork bits and expand them a bit.
Steve