Theres now 3 Different types of Mobil 1, Bare with me here.
0w-5, 15w-40(Original) and 10W-50(I Think).
As I've read from other Astra Owners, They don't like Mobil 1 too Much.
It causes Valve Train Chatter on Start up till Warm.
I'm using Castrol Synthetic R 15W-50) and I don't have no real problem except when cold its sluggish coz the Oil is Thick till Warm.