Lower number being less viscous when cold and higher number the thicker when hot, the 10/40 is more versatile however marine engine oil has a spec unused in normal motor oil that prevents foaming and breakdown after long periods of constant high revs. This is important for outboards.
Arguably more important than the grade is whether it is synthetic or not. Synthetic oil has molecules of a consistent size making it outperform regular oil and is one of the rare examples where man makes it better than nature.
The Standard Quicksilver 10w-30 is the advised oil for your OB for general use... from -18 degC to +40 degC which will pretty much cover use all around the UK/Ireland area. It doesn't state if it is synthetic or not but for this use that's less important than the fact it's what the makers advise.