On Sun, 07 Jun 2015 18:35:58 +0100, Jim at the Common Riding wrote:

but I'd want to find out if there was a particular reason that
viscosity was recommended. 5w30 is REALLY thin for a '90s lump.

Parts of Canada can get awfully cold in the winter...

no... no... that is what they call the engine...

Because it was developed and built there. So, yes, the recommended oil
may well be specced for cold winters and - in a car sold in the south of
the US - hot summers.