Just go to a motor factor and get their budget fully synth as Aragorn says - its about £36 a gallon and good, especially when changed regularly enough. Mine is getting very dark after 6k so needs doing again, make sure you get a new sump bolt (audi/vw only) and the correct filter if your doing a change.
As far as I know they are only available by mail order, cost about £25, but well worth it. Can give you the phone number if you want it. From a place in London that only accept cheques and served by a very posh bird called Phillipa!!