BMW MINI engine oil grades

[a2z]

The answer to this is probably very simple but it escapes me. Just picked up a Mini One yesterday and flicking through the manual there is no proper reference to the engine oil used. It refers to the BMW longlife oil but not the grade (e.g. 10/40 semi synthetic) All we need is a small bottle of Castrol to keep in the car if we need to top it up.

Does anyone know the proper grade to use? Thanks.