Just buy the stuff in 25 litre containers, split it with your mates if needs be. Or go to the local motor factors and haggle a bit. As per the story above, I generally get oil meeting VW spec (not the fully synth stuff mind) for approx £1 a litre when buying 25L at a time, and that's from the factors, not the oil firm. For what it's worth, that oil is above minimum spec for my 20 year old Golf and my girlfriends' 12 year old golf.
I really wouldn't run cheap oil in any car on the basis that you change it more often – by the time you account for a new sump plug, filter, washer & your time emptying the sump, refilling it and taking the oil to the tip, you've saved tuppence and risked your engine's health. The Asda 20w50 would be below spec for my 20 year old Golf, and probably anything short of a Morris Minor…