I remember fuelling up in back road France, after which the old thing pinked at almost any use of acceleration, until we were able to mix in some better quality petrol some time later.

As for book settings..I suspect they were printed relative to the fuel quality of the time..? I think that these days there are so many fuel types out there which the authors of the handbooks/manual may never have contemplated, if so then going by the book may be less than ideal...?