2009 9-5 Turbo Edition...Best Oil?

[highway]

I was going to take advantage of the current Halfords offer and buy some. I assumed Mobil 0/40 The web tool suggests Mobil 1 ESP 0W40 which is a fair bit more expensive. Whats best?

[highway]

I've read a GM specific 5w 30 is best rather than ow 40 mobil one ?