I have heard talk about possible negative effects of using modern oils in engines, because of certain additives that were included or left out. 
Trying to look into it, i found this page on the Penrite website.
Penrite Oil Company website home of Penrite lubricants
Unfortunately, my own thickheadedness is slowing me down to reaching a conclusion.
Does anyone know much about this idea? Penrite Australia offer 'vintage-style' oil to suit classic cars, but would it make much (if at all?) difference to a petrol 2.25?