Synthetic Oils aren't always best for normal engines, they tend to be designed for high performance needs, same goes for differences in viscosity. If you change from a synthetic to a semi or non synthetic, give the engine a flush.
I run my road cars on Castrol Magnatec, it's more than good enough, a friend sells Castrol oils and talked directly to the Castrol technical department on this.
Older engine designs especially require a higher wighting (ie thicker) as the machining tolerances are poorer (they don't fit together as good as a modern engine).
I know a race engine builder who uses Castrol RS in full race motors, he thinks Mobil 1 is pants, but as with all matters like this, it can be down to personal experience.....