Went to makros and saw Castrol GTX oil 5 Litre for £12.64 inc VAT, normally in halfords they are £20. They had loads left.
They have the high mileage ones and modern engine ones. The high mileage one say its for both patrol and diesel.
By the way at Makros i saw on the price label that the deal should have ended on 06/06/2009, but however its still running. Best thing to do is get yourself to Makro ASAP!!!