Just to add - if you know someone with a Costco card, they sell Mobil 0w40 'New life' for £29.89 for 5 litres.
I have this in my car currently. Previously had Castrol Edge 5w30, which is £35 from ASDA, but its only 4 litres.
Castrol didnt need topping up over 6000 miles, where as the Mobil had needed topping up once. But that may be because the Mobil has exclusively done motorway speeds and the Castrol did local miles.