Don't stress Kakaire... I'd rather change my oil regualrly (in an old car) than use expensive oil (it gets pretty polluted in in an old engine). Unless you recycle (filter) the oil yourself it is not cost-effective using synthetic oil in an old engine, (yet).

It's worth it in the diff, power steering and gearbox though- as it doesn't get polluted there and you can clean them out properly before putting it in... Nick