My 1st gen has been using 0w40 Mobil 1 for over 100,000 miles. Seems to like it just fine. Forgot to mention at each oil change a bottle of Chevron Techron goes in the tank.
We tend to get hung up on oil viscosity numbers as measures of performance rather than the equally (or more) important API and ACEA specifications. Here's a nice little summary: Oil Labelling Explained - TechWiki Note that Mobile 1 0w-40 meets both the ACEA A3/B3 and A3/B4 specs and has very acceptable HTHS (though it's not the highest on the list). That's why it's listed in the MINI manual and why many of us choose to use it.
Just use 5w30. I really don't want to start an oil debate. I believe when you use 0w40 you get into oil consumption problems. All the new manufactures that use it make allowances for consumption. The reason for 0W40 is fuel efficiency. This is the opinion of an old guy and worth what you are paying for it.
Sometimes the old guys have the good info. Its normally wise to listen to them. Unless they go off tangent and start telling a story about how they walked up hill both ways to school in 10 feet of snow with no shoes on and they liked it.