Both the 5w and the 10w won't make too much difference in most places, however, there are large parts of America that get much colder than both Japan and northern europe, including England.
That's the reason for the 5w, the 30 is what changes anything under normal running temperature.
I wonder if in nominal temperature areas, a synthetic (more free flowing at the same temperature and same 'weight' than conventional oil) oil at 10w instead of 5w, would sit on the top of the engine longer, aiding cold starts?
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