4.5L Smart Price 20w/50 Motor Oil - £2.12 (normally £4.24). Spotted in Asda Living, so not sure if this extends to Asda stores (it is still £4.24 online). They also had some other Castrol GTX 2l bottles half price but can't remember the details. Think it was diesel 10w/40 for about £6-7.
I know this deal will get voted cold and bring out loads of people saying the oil is crap (which I would probably agree with as to start with it has no official ratings, like ACEA, whatsoever on the can) - but if you have an old banger that drinks the stuff nearly as fast as petrol, and is probably off to the scrapper soon, I doubt you would find cheaper.