Im about to do an oil and filter change on a 1'4,8 valve GP.
Historicaly I have always used 5/40 but Im considering using 10/40...
Its on 80k and doesnt leak,smoke or obviously loose oil.
Engines relatively quiet and smooth. Its not thrashed around,more like its not thrashed enough imho. Short journeys mostly of perhaps 2 miles each way.
So the question is..10/40 ok to use?

No.

Use what it's meant to.
Especially if it only does short journeys get the thinnest it can use.

In general yes, pour the type suggested in the manual for particular country/continent.
But it's not so obvious, what is the best for an engine.
It depends on: climate (temperatures), local law (emissions), fuel (gasoline + alcohol?).
Look at Brasilian market: latest 1,4 8V engine should run on 15W/40 there!
From official manual (modern "UNO" model).