Possibly something in the hydraulic tappets issue, though I dunno if iit applies to this engine, and even if it does, it may be the lesser of the evils if the engine has sustained bearing damage.
I'm afraid the rest of it is (IMO of course, but based on quite a lot of reading and discussion) fairly standard myth with little visible support.
In particular, within the pumpability limits of the oil, which aren't going to be exceeded in the UK, a standard positive displacement oil pump delivers an almost fixed volume irrespective of oil viscosity.
I say almost because I think I've read somewhere that a worn pump will deliver slightly less of a low viscosity oil because some of it drains back.
Re specifying a thicker oil for an older engine, thats of limited commercial value to the maker, and they probably wouldn't be able to predict how worn an engine of a given age would be anyway, still less an engine that has seized.
If they don't do it, that doesn't mean it isn't the right thing to do, especially in a potentially extreme case like this one.
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