Remember most manuals are written by the MFG and could protect them more than help the user.
Most tell you to use castor so that the engine is protected more when ran hot and lean. They have to write that manual to host a range of people from beginners to experts. They lean more on the beginner than the expert. If they told what to run for best power and expect you to tune correctly then they would. Now if the new guy ran the engine lean burned it up and wanted a warranty exchange they would go broke.

I find it funny how people talk about thinner and thicker oil saying this or that but its mixed in with methanol and the viscosity is much closer to that of the methanol lol.