Have a look at Opie Oils for deals on Castrol Edge 5W40 - they usually have free shipping deals and if you are a member of ANY of the free to join forums they recognise then the prices are much lower.
Just had a quick butchers and 4x 4 litres is £152.53, so £38 a litre - depends what you want really. Not a fan of longlife service, but if it's cheap enough to change as frequently as regular oil then not really an issue
