I read an article done by Cambridge university (I think) They tested Mobil 1 against an 'average' priced oil and the cheapest supermarket brand (all oils were the same oil spec)
From 0-2000 miles the lubrication/viscosity of all the oils were nearly identical and all degraded at about the same rate. From 2000-10000 were where the results really changed, Mobil 1 pretty much stayed the same and the other two slowly dropped off through the test, with the cheapest supermarket brand dropping the most! The tester did comment that although the cheapest brand had dropped the most, it still wasn't critical to lubrication/protection.
I will see if I can find the website, it was quite interesting!!
From what I could gather, if you had a car that you just used as a run around, then cheap stuff would be just fine as long as you didn't leave it too long between changes!
Not the site I was looking for but very interesting
http://http://www.mobil1.com.sg/performance/miles.aspx